Page created by T Dec 31st 2010:
The Unofficial Guide to the
Rochester Varajet II
Covering as much of the 2SE and E2SE as I could find information for
All material is the property of its respective owner (and rusty borrowed it as the geocities site keeps turning it off;)
This introduction is a compilation of a couple Motor manual introductions, the Haynes Rochester Carburetor rebuild manual, the Doug Roe Rochester Carburetor book, and some stuff that I researched.
Fuel economy standards enacted in 1978 forced the realization that smaller engines in downsized cars were a must. The Dualjet's utility was being stretched to the limit on medium-size, low-performance GM vehicles.
A new carburetor was designed for smaller engines. The two-stage carburetor, built almost entirely of aluminum, first came out on GM front-wheel-drive vehicles. The Varajet would serve all GM divisions (see usage chart at the end) using the 2.5-liter four-cylinder and the 2.8-liter V6 until throttle body and port fuel injection technology made conventional carburetors obsolete. It was also used on the 1.8L four cylinder engines, and by the Holden division of General Motors (Australia).
The first model (1979) 2SE didn't have a Computer Command Control (CCC) system with a mixture-control solenoid and electronic idle-speed control. It was introduced as a conventional downdraft carburetor. The electronics were added within two years.
Why did RPD (Rochester Products Division) go to the expense of designing a new model for small engines when they already had the Monojet with its good track record? For the same reasons the Dualjet replaced the 2G - the idle, off-idle and main system metering couldn't be controlled enough to meet emission and economy standards. If the Monojet were downsized enough, and the venturi made sensitive enough to meet emission and fuel-economy standards, It would have been too small to deliver adequate HP.
The Varajet models 2SE and E2SE, Figs. 1 and 1A, are two barrel, two stage, down draft design carburetors. Aluminum die-castings are used for the air horn, float bowl and throttle body. A heat insulator gasket is used between the throttle body and float bowl to reduce heat transfer to the float bowl.
Rochester Varajet 2SE picture from Motor Manual (ISBN: ) This one would be figure 1.
Rochester Varajet E2SE picture from Motor Manual (ISBN: ) This one would be figure 2.
The 2SE two-stage was designed with a triple-venturi 35mm primary bore for fuel metering control during idle and part-throttle operation. That bore was too small to expect any reasonable HP, so a 46mm secondary bore was added. This design supported the power requirements at heavy throttle.
An air valve is used in the secondary stage with a single tapered metering rod. Metering control is governed by the air-valve opening so a suitable power mixture prevails regardless of how far open the secondary is.
The float chamber is internally vented though a vertical vent cavity in the air horn. The float chamber is also externally vented through a tube in the air horn. A hose connects this tube directly to a vacuum operated vapor vent valve located in the vapor canister. When the engine is not running, the canister vapor vent valve is open, allowing fuel vapor from the float chamber to pass into the canister where the vapor is stored until normally purged.
An adjustable part throttle screw is used in the float bowl to aid emission control. This screw is factory pre-set and a plug is installed to prevent further adjustment or fuel leakage. The plug should not be removed or the screw setting disturbed. If float bowl replacement is required, the service float bowl will include a factory pre-set and plugged adjustable part throttle screw.
A hot idle compensator is used on some models and is located in the air horn. The opening and closing of the hot idle compensator valve is controlled by a bi-metal strip that is calibrated to a specific temperature. When the valve opens, additional air is allowed to bypass the throttle valves and enter the intake manifold to prevent rough idle during periods of hot engine operation.
The idle mixture screw is recessed in the throttle body and is sealed with a hardened steel plug to prevent alteration of the factory pre-set mixture setting. The plug should not be removed and the mixture screw readjusted unless required by major carburetor overhaul or throttle body replacement.
Another feature of the 2SE is its low-profile design. On modern cars, engine compartments are smaller and hood lines lower, so tall units are out of the question.
The E2SE carburetor, includes special design features for use with the Computer controlled Catalytic Converter System (C4) or the Computer Command System. An electrically operated mixture control solenoid mounted in the air horn, controls air and fuel metered to the idle and main metering systems of the carburetor. The plunger located at the end of the solenoid is submerged in fuel in the fuel chamber of the float bowl. This plunger is controlled by an electrical signal from the Electronic Control Module (ECM). The Electronic Control Module responding to signals form the oxygen sensor in the exhaust and other engine operating signals, energizes the solenoid to move the plunger down to the lean position or de-energizes the solenoid to move the plunger up to the rich position to control fuel delivery to the idle and main metering systems. When the plunger is in the lean position, fuel metering is controlled by a lean mixture screw located in the float bowl. When the plunger is in the rich position, the additional fuel is metered to the main fuel well through a rich mixture screw located at the end of the fuel supply channel in the float bowl. Air metered to the idle system is controlled by the up and down movement of the mixture control solenoid plunger. The plunger increases or decreased air supplied to the idle system which is further metered by the idle air bleed screw. The plunger cycles up and down approximately 10 times per second, controlling air and fuel mixtures.
On 1981 models with 4-151 engine and air conditioning and 1982 models with 4-112 (1.8L) engine, an idle speed control motor which is controlled by the Electronic Control Module is used to control idle speed, Fig. 1B. The curb idle speed is programmed into the Electronic Control Module and no attempt should be made to adjust idle speed using the idle speed control motor.
CAUTION: On 1980-83 units, use care not to remove the special friction reducing coating applied to the primary and secondary throttle shafts, the secondary actuating lever and lockout lever. On 1980 V6 units and on all 1981-83 units, a special graphite compound is also applied to the secondary throttle bore and valve.
Rochester Carburetor Rebuild Tools:
Float height gauge
Choke Angle gauge J-26701-A or drill bits
Bending Tool J-97789-111 or flat-nose pliers
5/16"-18 x 4" Carriage bolts
Other tools needed:
Safety Goggles!!!!!!!!!! WEAR THEM WHEN USING CARBURETOR CLEANER!!!!!!!!!!!!
T-10, T-15, T-20, T-25, T-27, T-30 Torx driver bits
5/16" flat blade screwdriver for needle seat
3/16" flat blade screwdriver
#2 offset Phillips screwdriver to adjust the idle
A digital or analog tachometer
Needle nose pliers
9/16 flare nut wrench
1" open end wrench
Ratchet (Note: 1/4" drive may fit better)
Carburetor cleaner spray
5mm x 0.80 tap and die
6mm x 1.00 tap and die
Removal and Installation:
EFE Grid (from How to Rebuild your GM 60 V6)
If your engine is fitted with a carburetor, An Early Fuel Evaporation (EFE) grid is probably used underneath it. This grid preheats the fuel as it flows from the carburetor to the inlet manifold, thereby improving driveablity when the engine is cold. Unfortunately, this grid has a tendency to disintegrate and cause driveability problems.
After sliding the EFE grid off the carburetor mounting studs, carefully inspect it. The bars forming the grid should not show signs of erosion, and the grid should be intact for the entire opening in the plate. If the grid has any problems, replace it.
Caption If you car or truck engine is equipped with a carburetor, and is a 1981 or newer model, and Early Fuel Evaporation (EFE) grid is used underneath the carburetor. Eventually, the EFE grid breaks up, causing a host of driveability problems. If yours is missing any part of the grid, replace it.
Stripped fuel inlet
Most Rochester carburetors are made of "pot metal".
Karl not know what that means, but it is not as strong as steel.
As a result, a notorious problem is stripped threads on the inlet.
There is a solution for this, but it is not pretty. There is a part available from
GOOD auto parts stores. It is an oversized, self-tapping, inlet fitting marketed as a
"HELP!" or "Perfect Parts" product.
You install it by forcing it to cut new threads into the fuel inlet.
It has been suggested by Doug Kichener that these fittings not be removed as there are likely
to be small metal fragments held between the threads by the new fitting. Removing the fitting
could permit these bits to get into the carb and catch in the tiny channels which flow fuel.
This is a good time to install an inline filter into the fuel line.
A new line is probably necessary anyway as the new fitting can be more than an inch long, which will
be a problem for any factory original steel tubing!
Disabled EGR system.
Neither Karl, Rochester Products Division, or anyone else contributing to this site advocates
or in any way recommends altering or disabling your emission control equipment in any way.
That said, those whose EGR systems have "mysteriously" ceased to function have noticed that
their vehicle runs differently when EGR is non-functional.
The reason for this is that the EGR pump operates on manifold vacuum.
The primary metering rods are raised/lowered into the jet orifices by a piston that responds
to the manifold vacuum signal. When the EGR is inoperative, the manifold vacuum signal
available to the carburetor is very different from the conditions that the metering rods
and jets were designed for! This typically results in a lean condition, possibly including "surging".
The only real solution is to fix the EGR system.
Those incorrigibles out there who refuse to do this have to have their carburetor re-jetted
and re-rodded to correct for this. Best just to use rods and jets from a pre-EGR carburetor of
the same displacement.
Hopefully, Karl will be able to provide the appropriate part #s here in the future.
On a similar note, if the ECM (computer) is disconnected from an appropriate-year
Quadrajet, a severely rich condition results. Again the only real cure is re-rodding.
Needle and Seat
This is addressing a problem that Karl has had with his 1965 Rochester 2GC.
The symptoms are a periodically terrible idle that may have coincided with an instance of
WOT and may similarly go away after another instance of WOT. The motor will sputter and die
at idle and in fuel will contimue to flow from the discharge nozzles after the engine has
died. Sometimes fuel will leak through the throttle plate hinge shaft holes to the outside
of the carburetor and onto the manifold!
This has been diagnosed for me as contamination preventing the needle from seating properly
and causing a flooding or overfilling of the float bowl.
The cure for this is to rebuild the carb and replace or adjust the needle and seat.
Remove any debris from the float bowl and check or replace the fuel filter.
Use the right gasket. It may not be an exact match due to changes in the design of the gasket, but it should still have the same holes blocked/unblocked the original one did or you WILL have problems.
Clean the carburetor. Try dipping it in a can of carburetor cleaner and brushing it off. Then spray it with some spray cleaner. Then dry with compressed air.
DO NOT USE THE CLEANER NEAR OPEN FLAME. WEAR SAFETY GOGGLES. ONLY PUT METAL PARTS IN THE CLEANER BECAUSE IT IS VERY HARD ON PLASTIC (99% CHANCE IT WILL MELT PLASTIC). IF IT WILL DISSOLVE PLASTIC, IMAGINE WHAT IT COULD DO TO YOUR EYES, SO WEAR SAFETY GOGGLES. USE THE CLEANER IN A WELL-VENTILATED AREA. DID I MENTION YOU SHOULD WEAR SAFETY GOGGLES?
If a vacuum line looks bad (cracked, dry-rotted, etc ) replace it.
Make sure the float level is correct. Too low a level causes a lean mixture, resulting in late or hard starting, a flat spot in acceleration, or stumble. Too high a level causes a rich mixture, resulting in the engine racing after starting, percolation, and even fuel pouring out the bowl vents.
Some parts need to be soaked overnight, brushed, and then sprayed off. You should wear safety goggles when you do that
I have heard of two jet/metering rod sizes so far. #174 from high-altitude carburetors (I think) and #180 from others.
Rochester Varajet carburetor quick-takes.
Motor Age, Jan 1983 v102 p28(2)
Full Text: COPYRIGHT 1983 Chilton Company; pictures not available
FIELD FIXIN' FINICKY FUELERS
On some of these Varajet secondary vacuum break diaphragms (circle), you have to grind off or cut off the cap on the end of the diaphragm before you can reach the adjustment screw inside it. Remember to find the air bleed hold in the diaphragm and tape the bleed closed before you pump it down for the ajustment.
Because we have gotten as many complaints about air horn screw location as we've gotten about the linkage on this carb, here (at left) is your own screw locator chart [chart not available]. Tighten screws in the numbered sequence.
Little one-use, throwaway clips adorn the Varajet's linkage. If the tip or end of the linkage points outward (arrow), the teeth or raised edges of the clip must point outward as shown here. If the end of the linkage points inward, clip's teeth point inward too. Don't press the clip on so tight that it binds up the linkage! New clips are included in carb overhaul kits. Instead of growing yourself a third arm, try taping the choke blade closed during choke linkage installation.
Oh horror! Secondary metering rod can fall out and sometimes does fall out of place. The stray metering rod may just create a severe bog or hesitation and then lay inside the secondary throat of the carb. Other times, it slips past the secondary throttle blade, falls into the engine, and creates a different kind of problem! When you install the metering rod, pull back the spring on the rod (circle) and be sure that the rod snaps firmly into the little bracket on the air valve.
Remember to dab some grease on that air valve spring where the arrow is pointing. Remember that some Varajets have a Q-jet style air valve adjustment screw and lock screw at the end of the air valve shaft, some don't.
If you are extremely careful, GM's procedure for hacksawing slots alongside the mixture plug and knocking the plug out with a punch words okay. But if you are slightly un-careful, you'll crack the throttle body--the base of the carb! Experience has taught us that the safest, easiest way to remove a mixture plug is to cut around the plug with a holesaw-type tool such as the Thexton 353 or the Borroughs BT-8211. Take your time, these carbs are expensive!
Whenever you get a Varajet-equipped engine that surges or runs poorly during light-throttle or part-throttle driving, check basic engine condition, check for fault codes in the computer control system, and then check the mixture solenoid dwell at 3000 rpm in neutral. Dwell at 3000 must average 35 |. Grab a Borroughs BT-7928 or a Kent-Mmore J-28696-10, the same mixture tools you use on the Dualjet and Q-jet. Remove the vent screen from the air horn and slide the tool into the lean mixuture/lean authority screw shown here.
Holden division of GM
VB - VK Substitute Carburetter (Black and Blue Motor)
The most common cause of engine troubles with the 202 motors was the Varajet carbi. It made the engines work to well bellow their potential. A cheap substitue for the varajet is a small Weber (32-36) off of the XE falcons. They cost around $40 dollars (plus adaption plate) and are easily fitted to these motors with only the accelerator cable mount needing to be modified. They not only provide greater horse power but also increase your fuel effeciency. (N/B: With the black motor in the
VK the secondary throttle sensor must be disconected and the computer (EST) must be reset.
From: Kris Humphrys
VH (possibly) and VK 3.3L Carby's flooding
If your sitting at the lights and your VH/VK stalls on you and a strong smell of petrol is hanging around, more than likely it is a stuck needle valve in the carby. One solution is to dismantle the carby and rekit it, or another simple and less expensive option is to take the air filter off, and where VARAJET is written, tap on it with a screwdriver. This will move the needle so it sits back in its seat.
From: David Redmond
Carburetor Kit Application Guide
Carburetor Kit Contents
Inside the Carburetor
Exploded View Diagrams
Overhaul and Adjustment
Use the numbers as the tightening sequence for the top screws.
5mm x 0.80
5mm x 0.80
5mm x 0.80
5mm x 0.80
5mm x 0.80
5mm x 0.80
6mm x 1.00
6mm x 1.00
C. Feedback Feedback: Those Troublesome Electronically Controlled Carbs
Call it a compromise, stopgap, interim solution, or crutch, the feedback carburetor was never all that satisfactory, a relic of the mechanical age trying to make it in the electronic era. Quaintly obsolete, it had all the frailties and drawbacks that carburetors are heir to, with the additional complications of a remotely controlled mixture system. The number of vacuum and electrical connections to the carb became downright ridiculous. The confusion of the underhood environment was enough to make any technician dream of early retirement. Just compare the engine compartment of an early-80's feedback carburetor-equipped car to that of a later model with EFI. That's progress we can all appreciate.
Speaking of injection, for many, many years I've been predicting that that supremely accurate means of metering gasoline would supplant the carburetor. It's finally happened -- all new vehicles now sold in this country are so equipped. But huge numbers of cars using feedback carbs are still on our roads. One thing can be said for them: They got us through a difficult transition period to more stringent emission standards, providing good enough mix control to allow the three-way cat to its job.
When you trace a problem to an electronic feedback carburetor, you won't be doing yourself a favor by just automatically replacing it. Not at these prices -- a typical two-barrel lists for an earth-shaking $800! Even remanufactured versions cost maybe $500.
Given these expensive circumstances, there's little doubt that you'll be fixing these late model carbs whenever possible rather than just taking the simple route and bolting on a new or refurbished unit.
Biting the bullet
Yeah, I know. The whole subject is intimidating. I have to admit to a little trepidation the first time I took one apart. But I found that once I'd disconnected the, oh, two or three hundred vacuum lines and electrical connections, and had the carb on the bench, it didn't seem so scary or mysterious anymore. Just a modern carburetor with some logical-looking additions.
Fortunately, many of the basic troubles that mechanical air/fuel mixers have traditionally been heir to (say, a soggy float, a leaky needle and seat, or a faulty accelerator pump) will still be repairable with the same general procedures you've always used. Some other good news is that a lot of the stuff I'm going to talk about here can be serviced with the carb mounted on the engine.
On the other hand, there's some new hardware to deal with, such as mixture control solenoids, throttle position sensors, temperature-controlled accelerator pump bypasses, stepper motors, idle air control valves, etc., and a number of unfamiliar adjustments too. Since I can't give you detailed info on all the specimens out there, I decided to concentrate on one of the most common electronic carburetors in our vehicle population: the Rochester VaraJet II (E2SE), a staged two-barrel which you'll find perched atop certain GM engines (my example came off an '84 2.8 liter Chevy V6). I'll give you a quick course on a few of the jobs you're likely to run into.
A note on electrical readings: You can get the necessary data remotely through the ALDL (Assembly Line Diagnostic Link) using a scan tool, but I'll give direct tests, for which you'll need a digital multi-meter with sufficient impedance to ensure against frying the ECM, and an ordinary dwell meter (hey, who'd have thought you'd ever need that again?).
As has always been the case, before you start tampering with the carburetor, make sure you're not bearing false witness against it. In the past, this meant a simple examination of the ignition system and maybe compression. Now, of course, unless the problem is obviously in the carburetor (an overflowing bowl, perhaps, or a lack of accelerator pump output), you've got to check out the sensors and the other electronics (in this case, the C-3 system) before you condemn a carb, but that's another story. Suffice it to say here that if the computer and its network of assistants and wiring are okay, there are no vacuum leaks or linkage troubles, and emissions controls such as the heated air intake and EGR are working properly, the carburetor is the likely culprit.
I should mention two preliminaries: Make sure you've got the proper Torx drivers, and put some feet on the mounting flange ears to protect the throttle plates (four bolts with two nuts each work fine).
Now, suppose you're confronted with symptoms that point to a faulty air/fuel ratio. Do you start by breaking out the steel anti-tampering plug and twisting the mixture screw? Not if you're smart you don't. That factory setting is probably correct, and if you don't want to go through a lot of gyrations trying to get it back where it belongs, look into more likely causes of the condition. Clogged passages could be the problem, but you check for them in every carburetor you disassemble, right? That's nothing new, but electronic mixture control is, and it's easy to test. Providing you got the proper dwell readings during your initial checkout, the mixture control solenoid itself should be suspect. Proceed as follows:
Remove the solenoid from the air horn and take the rubber seal, retainer, and spacer off the end of the solenoid stem.
Attach a hand-operated vacuum pump to the solenoid stem, and then connect a jumper wire between either electrical terminal and a 12-volt positive source. Hook another jumper between the second terminal and negative.
Apply 25 in. Hg. The factory info says to time the leak-down rate from 20 to 15 in. Hg., and that if it loses those five inches in less than five seconds to get a new solenoid. I hate to disagree with holy writ, but it's been my experience that perfectly good specimens leak down very quickly once the vacuum reaches 20, so I time them from 25 to 20, which they've all done in about five seconds.
Remove the jumper wires to de-energize the solenoid and watch the vacuum gauge. It should go to zero in less than one second. If it doesn't, it's sticking in the down position and should be replaced.
If the solenoid's okay, check the metering jet for looseness, damage, or dirt.
While I strenuously recommend against tampering with the mechanical mixture settings, there will be cases where all your testing brings you to the unfortunate but unavoidable conclusion that they must be changed (maybe somebody else has fooled with them), and there's only one way to do it right:
With the manifold side up, saw two parallel slots in the throttle body so that they meet the idle mixture screw plug.
Using a flat punch at a 45-degree angle, crack off the tab you've created in the casting, then break out the plug.
Turn the screw in until lightly seated (it has a "Double-D" head that can be turned by wedging a screwdriver in next to it, but it would make things easier if you either buy a special tool, or fabricate your own by partially flattening a piece of tubing or cutting a notch in the end of a 1/4 in. bolt), then back it out four turns.
Remove the screen from the vent stack in the air horn to get at the lean mixture screw (it too has a "Double-D" head, but smaller). Seat it, then back it out two and a half turns.
Mount the carb on the manifold, but don't connect the bowl vent line or the EGR and canister purge hoses. Instead, cap the ports on the carburetor. Plug the port that supplies vacuum to the air cleaner temperature sensor also.
Connect the + clip of a dwell meter to the mixture control solenoid's blue wire (slip a stiff wire into the back of the plug as a tap), and the - clip to ground. Set the meter for six cylinders. Hook up a tach too (use the brown distributor leads).
Run the engine until the coolant fan comes on.
Hold the rpm at 3,000, then watch your dwell meter while turning the lean mixture screw -- you want 35 deg. If it's too low, back the screw out, and vice versa. You'll get a reading drift of 5 deg. with occasional excursions to 15 deg., but you're looking for the average. If you can't get it right, the main metering circuit probably has a leak or restriction.
Let the engine return to idle, and adjust the idle mixture screw until you get an average dwell of 25 deg. while the coolant fan is off. Back the screw out to raise the reading.
As a check, unplug the mixture control solenoid connector while observing your tach. If rpm doesn't change by at least 50, the idle air bleed circuit is clogged or leaky.
Reconnect the M/C solenoid, run the engine back up to 3,000 rpm for a few seconds, then note dwell. It should average 35 deg. If not, reset the lean mixture screw, then the idle mixture screw. A little RTV will work fine to seal up the idle mix screw cavity.
The Throttle Position Sensor (TPS) tattles to the computer about what the driver's doing with his right foot. It's another thing you shouldn't meddle with unless you have no choice. But if the TPS is defective, or you have to replace the air horn or float bowl, adjust as follows:
Drill a 5/64th-in. hole in the steel cup plug that covers the TPS adjustment screw. Even though I'm pretty careful, I've cut into the screw head, so watch it.
Using a small slide hammer or a self-tapping screw and two screwdrivers for leverage, extract the plug.
Disconnect the TPS plug and run three jumper wires between the plug terminals and those of the TPS.
Attach a digital voltmeter across the center terminal (B) and the bottom terminal (C).
With the ignition on and the engine and A/C off, turn the adjustment screw until you see .26 volts on your meter.
Remember, this is an important setting to the operation of the C-3 system, so get it right. Seal the adjusting screw cavity with a new plug or RTV.
If the choke heater connection is getting 12 volts with the engine running, but the choke isn't opening properly, it's an easy matter to drill out the choke cover rivet heads with a 5/32 in. bit and replace the part. You'll have to make an extension for your pop rivet tool for installation. Use a piece of 1/8-in. steel tubing one inch long. By the way, if voltage to the choke was low or zilch on a vehicle which has a temperature/oil pressure warning light instead of gauges, and the wires and connections are okay, chances are you need a new oil pressure switch.
The float setting, fuel filter, idle stop solenoid, etc. are handled pretty much in the normal manner, and idle speed should be adjusted according to the Emission Control Information Label.
General Motors CCC Carburetors: Part I
Starting with the 1980 model year on California models, and with 1981 on 49-state models, General Motors introduced a new carburetor system on its B-, G-, F- and J-body cars. Since then, this system has confounded many a shadetree mechanic, and been cussed at, cussed up, and cussed out more times than anyone cares to count. My aim here is to provide a little understanding of exactly what this system is about.
Let's start with the QuadraJet. Back in 1965, General Motors replaced its 4G four-barrel carburetors with a new design, the model 4M QuadraJet from its Rochester fuel-systems division. This carburetor is a downdraft two-stage spreadbore design with vacuum secondaries. The primary rods and jets can be changed for various sizes and styles, and so can the secondary rods. The secondary jets are machined into the float-bowl casting. The carburetor is cast of two pot-metal parts, the air horn and the float bowl, and one aluminum part, the throttle body.
There are a few variations of the QuadraJet. The 4M has a manual choke. The 4MV has a divorced automatic choke, a thermostatic coil on the intake manifold. The 4MC uses a hot-air automatic choke. The 4ME has an electrically heated choke coil, and was invented to hurry up the choke opening to meet stricter emissions standards in the late '70s.
In the mid-'70s, GM chopped off the secondary-side barrels of the 4M QuadraJet to come up with the model 2M DualJet. The DualJet is just like a QuadraJet with two barrels missing. It's very easy to think of it this way.
The carburetors you are likely to run across on 1981-on G-bodies are the E2ME DualJet and the E4ME QuadraJet. The "E" prefix designates that this is a feedback carburetor.
What is feedback, you ask? That means that the carburetor is merely a part of a computer system that runs on a feedback loop. The computer constantly monitors inputs from several sensors and makes decisions to change things based on these inputs. Its program is like a list of steps, and when it gets to the last step it always goes back to step 1.
What sensors does this computer monitor? There are several. I will now list them along with a description of what they do.
Engine Coolant Sensor: a thermistor (resistor that changes resistance based on temperature) stuck into the thermostat housing. It tells the computer about how hot the engine is running.
Manifold Absolute Pressure (MAP) Sensor: plumbed into manifold vacuum. It tells the computer how much vacuum is present in the intake manifold, which is a strong indicator of the load the engine is under.
Barometric Pressure Sensor (BARO): open to outside air. It tells the computer the ambient air pressure. Practically, this gives the computer a very strong indication of what altitude the car is at.
Differential Pressure Sensor: measures the difference between outside air pressure and manifold vacuum.
Oxygen (O2) Sensor: another variable resistor. This one changes its resistance based on the relative amount of oxygen passing through it compared to exhaust gas. This tells the computer how rich or lean the exhaust is.
Park/Neutral Switch: tells the computer if the car is out of gear.
A/C "On" Signal: tells the computer if the air conditioner compressor clutch is engaged.
Throttle Position Sensor: yet another variable resistor. It tells the computer just how far the throttle is open (or closed).
Vehicle Speed Sensor: uses a beam of light through a wheel of spokes to tell the computer how fast the car is going, based on the speedometer. Curiously, it uses the same technology as the typical desktop computer mouse.
Knock Sensor: tells the computer if there are strong vibrations around the crankshaft that could indicate a strong ping or knock.
Distributor Reference Pulse: computer counts these to tell how fast the engine is running.
Transmission Gear Position: uses this to decide if the transmission is in high gear, which is important for the torque converter clutch.
All these inputs are well and good, but what does the computer do with them? Does it just sit there like a bug at a White House cocktail party, soaking up information and never spitting any out?
The answer, thankfully, is no. The computer is able to control several things, as I will explain in part two of this article.
Which Engine Sensors Are the Most Important?
All sensors are important. The computer is the brains of a computerized engine control system and sensors are its link to what's happening under the hood.
Some sensors have more influence on engine performance than others. These include the coolant temperature sensor, oxygen sensor, throttle position sensor, and manifold absolute pressure sensor.
The coolant sensor is often called the master sensor because the computer uses its input to regulate many other functions, including:
^ Activating and deactivating the Early Fuel Evaporation (EFE) system such as the electric heating grid under carburetor or the thermactor air system.
^ Open/closed loop feedback control of the air/fuel mixture. The system won't go into closed loop until the engine is warm.
^ Start up fuel enrichment on fuel-injected engines, which the computer varies according to whether the engine is warm or cold.
^ Spark advance and retard. Spark advance is often limited until the engine reaches normal operating temperature.
^ EGR flow, which is blocked while the engine is cold to improve driveability.
^ Canister purge, which does not occur until the engine is warm.
^ Throttle kicker or idle speed.
^ Transmission torque converter clutch lockup.
The coolant sensor is usually located on the head or intake manifold where it screws into the water jacket. Sensors come in two basic varieties: variable resistor sensors called thermistors because their electrical resistance changes with temperature, and on/off switches, which work like a conventional temperature sending unit or electric cooling fan thermostat by closing or opening at a preset temperature.
Variable resistor coolant sensors provide the computer with a more accurate indication of actual engine temperature than a simple temperature switch. The computer feeds the sensor a fixed reference voltage of about five volts when the key is on.
The resistance in the sensor is high when cold and drops about 300 ohms for every degree Fahrenheit as the sensor warms up. This alters the return voltage signal back to the computer, which the computer then reads to determine engine temperature.
The switch-type sensor may be designed to remain closed within a certain temperature range, or to open only when the engine is warm. Switch-type coolant sensors can be found on GM "T" car minimum function systems, Ford MCU, and Chrysler Lean Burn systems.
Because of the coolant sensor's central role in triggering many engine functions, a faulty sensor (or sensor circuit) can cause a variety of cold performance problems. The most common symptom is failure of the system to go into closed loop once the engine is warm. Other symptoms include poor cold idle, stalling, cold hesitation or stumble, and/or poor fuel mileage.
The oxygen sensor (O2) measures how much unburned oxygen is in the exhaust. The computer uses this as an indication of how rich or lean the fuel mixture is so adjustments can be made to keep it properly balanced.
A problem with the O2 sensor will prevent the computer from keeping the fuel mixture balanced under changing driving conditions, allowing the mixture to run rich or lean.
The throttle position sensor (TPS) is used with feedback carburetion and electronic fuel injection (EFI) to inform the computer about the rate of throttle opening and relative throttle position. A separate idle switch and/or wide open throttle (WOT) switch may also be used to signal the computer when these throttle positions exist.
The throttle position sensor may be mounted externally on the throttle shaft (the case on most fuel injection throttle bodies), or internally in the carburetor (as in Rochester Varajet, Dualjet and Quadrajet).
The TPS is essentially a variable resistor that changes resistance as the throttle opens. It is the electronic equivalent of a mechanical accelerator pump. By signaling the computer when the throttle opens, the computer enriches the fuel mixture to maintain proper air/fuel ratio.
Initial TPS setting is critical because the voltage signal the computer receives tells it the exact position of the throttle. Initial adjustment must be set as close as possible to factory specs. Most specs are given to the nearest hundredth of a volt.
The classic symptom of a defective or misadjusted TPS is hesitation or stumble during acceleration. The fuel mixture leans out because the computer doesn't receive the right signal telling it to add fuel as the throttle opens. The oxygen sensor feedback circuit will eventually provide the necessary information, but not quickly enough to prevent the engine from stumbling.
When the sensor is replaced, it must be adjusted to the specified reference voltage. The TPS on most remanufactured carburetors is preset at the factory to an average setting for the majority of applications the carburetor fits. Even so, the TPS should be reset to the specific application upon which it is installed.
MAP sensor function is to sense air pressure or vacuum in the intake manifold. The computer uses this input as an indication of engine load when adjusting air/fuel mixture and spark timing. Computerized engine control systems that do not use a MAP sensor rely on throttle position and air sensor input to determine engine load.
Under low-load, high-vacuum conditions, the computer leans the fuel mixture and advances spark timing for better fuel economy. Under high-load, low-vacuum conditions (turbo boost, for example), the computer enriches the fuel mixture and retards timing to prevent detonation.
The MAP sensor serves as the electronic equivalent of both a distributor vacuum advance diaphragm and a carburetor power valve.
The MAP sensor reads vacuum and pressure through a hose connected to the intake manifold. A pressure sensitive ceramic or silicon element and electronic circuit in the sensor generates a voltage signal that changes in direct proportion to pressure.
MAP sensors should not be confused with VAC (Vacuum) sensors, DPS (Differential Pressure sensors), or BARO or BP (Barometric Pressure) sensors. A vacuum sensor (same as a differential pressure sensor) reads the difference between manifold vacuum and atmospheric pressure (the difference in air pressure above and below the throttle plate). A VAC sensor is sometimes used instead of a MAP sensor to sense engine load.
A MAP sensor measures manifold air pressure against a precalibrated absolute (reference) pressure. What's the difference? A vacuum sensor only reads the difference in pressure, not absolute pressure, so it doesn't take into account changes in barometric (atmospheric) pressure.
A separate BARO sensor is usually needed with a vacuum sensor to compensate for changes in altitude and barometric pressure. Some early Ford EEC-III and EEC-IV systems have a combination barometric pressure/MAP sensor called a BMAP sensor, combining both functions.
Anything interfering with accurate sensor input can upset both fuel mixture and ignition timing. Problems with the MAP sensor itself, grounds or opens in the sensor wiring circuit, and/or vacuum leaks in the intake manifold.
Typical driveability symptoms include detonation due to too much spark advance and a lean fuel ratio, and loss of power and/or fuel economy due to retarded timing and an excessively rich fuel ratio.
A vacuum leak can cause a MAP sensor to indicate low manifold vacuum, causing the computer to think the engine is under more load than it really is. Consequently, timing is retarded and the fuel mixture is enriched.
Case History: "Rough Idle, Just Needs a Tune-up"
An 85 Chevrolet Blazer was equipped with a 2.8 liter engine and a
Rochester Varajet carburetor. It had 52,000 miles on the odometer. The
customers complaint was a "rough idle" and "just needs a tune-up". The
Blazer "ran great" at highway speeds and had adequate power.
During the visual inspection, a large vacuum leak was found on the decel
valve hose. The original factory installed hose (3/8" I.D.) was "mushy."
This hose was tied into the PCV line. It was replaced with fuel line tubing
because of its durability.
Inspection of all vacuum lines continued. The evaporative canister purge
line hose was mushy, but not yet leaking. Someone had installed just
regular vacuum line hose, a no-no! This type of hose will not tolerate fuel
or fuel vapor. A new ¼" rubber fuel line hose was installed. I installed
these new vapor lines at this point because the leaks were making the
idle rough. I wanted to make sure I could detect other symptoms the
vacuum leaks might hide.
In my experience, a fuel line hose works very well and does not get
mushy with age. After repairing the vacuum leaks, the Blazer engine idled
much smoother, but it still wasnt right. A slight choking of the primary
barrel increased the idle speed 200 RPM. Thirty to 50 RPM would have
been OK, but a 200 RPM increase indicated the mixture was just too
The basic timing, total advance, cranking voltage, EGR valve and PCV
systems were all working properly and set at specification.
I measured and recorded the pertinent "scanner" information. The battery
charging voltage was normal at 14.5 with the lights on and 14.3 with the
air conditioning on. The coolant was 85° C. The carburetor mixture control
dwell was 38-42 at 3000 RPM in neutral. At idle, it was fixed at 30°.
The EGR control was 0% at idle and 99.9% off idle. This indicates the
EGR solenoid was off at idle and on above idle RPM.
The vacuum sensor was a normal 3.4 volts at idle in drive. The voltage
decreased as I opened the throttle; again, a normal reading.
The oxygen sensor voltage was 0.3-0.8 volt varying slowly (lazy) at 1200
RPM in drive. But it was fixed at 0.3 volt at idle (open loop!).
The throttle position sensor was set at 0.26 volt at idle (normal for most
Varajet carburetors). As the throttle was opened, the voltage increased
smoothly up to 4.8 (engine off, key on) at wide open throttle.
So, the only item at this point was open loop at idle and a "lazy" oxygen
sensor. Remember, the engine still idled rough even after repairing the
With the visual and running checks, and the scanner information, I offered
the customer new spark plugs, oil change, carburetor cleaning, gasket kit
and a new oxygen sensor.
The spark plugs were changed and a new oxygen sensor was installed.
The carburetor was "boiled out" and the new kit was installed. The float
level was set exactly to specification. The idle mixture seal was removed
according to the manufacturers procedure.
Before repair, the idle mixture screw was set at two turns out from the
seat. The idle mixture preset specification on this carburetor was four
turns out. The main jet was set at two and a half turns out (preset spec is
two and a half turns out).
I looked up the idle mixture specifications and discovered this application
(85 Blazer) had a "propane gain" specification as a way to check idle
mixture. The "propane gain" specification was 30 RPM above curb idle.
"Propane gain" is a method to see if the "lean drop" mixture adjustment
was done right. Tune-up manuals explain propane gain and lean drop
procedures and specifications for idle mixture adjustment.
On the 2.8 Litre Blazer, the curb idle specification was 650 RPM in drive.
The RPM propane gain meant the "best idle" was to be set at 680 RPM in
drive. The idle speed and "best idle" mixture was adjusted at 680 in drive.
The idle mixture screw was then turned in to drop the idle to 650 in drive.
After the lean drop was set right, I checked the number of turns out from
the seated position. The idle mixture screw was out seven turns! The idle
emissions were 0% on carbon monoxide and 15-25 parts per million on
The truck now idled very smooth and there was no off idle hesitation and
the emissions were way below the failure standards.
The dwell was still fixed at 30° at idle. It varied from 40-45 at 3000 RPM.
The car ran great and the customer was satisfied.
Many GM cars and trucks equipped with Varajet carburetors are
programmed to run in closed loop at idle. In this case (Blazer) the
"propane gain" specification for a truck was the clue to correct idle
mixture adjustment procedure.
The bottom line: A tune-up was performed, along with repairing major
vacuum leaks, installing a new oxygen sensor, and adjusting the idle
mixture [lean drop method] to specifications.
Prior to tuning the carburetor:
Make sure ignition is good, fuel pressure is adequate, and throttle is opening completely.
Check these things before starting to tune your carb:
Always make sure you have a clean air filter element, and check the amount of pleats your air cleaner has. The more it has, the less it will flow.
Check your ignition timing. Make sure it's not too advanced or retarded.
Make sure your distributor has the correct curve.
Make sure the vacuum advance is working properly.
Always install a fuel filter. Racers and high-performance buffs install their fuel filters ahead of the fuel pump, but street machines and daily drivers can install theirs between the pump and the carb with no problems.
For maximum output, make carburetor as large as possible - consistent with the required driving range
Using a lot of flow capacity can reduce inlet-system restriction and increase volumetric efficiency at WOT and high rpm
When you make the engine a better air pump, the torque peak and the entire torque curve are lifted to a higher rpm band. Usually, low rpm performance is reduced.
Dual-quad installations are not capable of providing usable low- or mid-range performance
Too small a carburetor will reduce power, because the pressure drop at WOT will be too large, and the mixture will not be as dense as necessary for full power.
Carburetors are flow rated at WOT, and the cfm rating is the amount of airflow that causes a specified pressure drop, due to air friction through the carburetor. The higher the flow rating, the bigger the carburetor. The bigger the carburetor, the lower the pressure drop across it.
Can the carburetor meter fuel correctly at low airflows?
Will it work well at the common speed range of the engine?
Accelerator pump shooter size has a direct affect on the initial off-line or "launch" performance
Too small: will produce hesitation and then pick up
Too large: may cause a bog or sluggish response from too much fuel, may produce a puff of black smoke on acceleration
To alter the shooter size, you'll have to obtain a set of very small drills (0.0135 to 0.040 inch) and a pin vise. Make changes only 0.002 inch at a time. When you have a clean, instant throttle response when the throttle is opened up under a no-load condition, you'll know you have the right diameter shooter orifices.
Duration of accelerator pump squirt
Gives a longer duration, but a weaker shot of fuel
Gives a shorter, but harder injection of fuel
The capacity of the pump can be changed by boring out the pump well and using a larger diameter pump cup, but this modification must be performed by a machinist specializing in carburetor modification.
Needle and Seat
Use Viton needle unless the type of fuel being used is one of the modern racing fuels where octane levels are increased considerably or when octane boosters are used in excess of the manufacturer's suggested levels
Inlet pressure should be between 7 and 8 psi at idle
More pressure may overpower the needle and seat and flood the carburetor
Inspect flat surfaces for warpage. Anything in excess of 0.010 will require resurfacing for a proper gasket seal. Draw filing is recommended with a medium-fine flat file to true the surface. Don't attempt to file any surface that has a raised sealing bead on it!!!!!!!!!!
If engine modifications have caused a low vacuum condition at idle, you may need additional airflow while keeping the throttle plates closed enough to prevent an overrich mixture. One way to do so, is to drill a 3/32 inch hole in each primary throttle plate. Note: If the engine is radically modified, it may be necessary to drill holes in the secondary throttle plates as well.
Stock jets are generally about right for most applications. That varies with the extent of engine modification.
The first changes that should be made to the air/fuel ratio in the main system should be made with the jets, or on models that have metering rods, the jets and metering rods.
To provide a better response as the secondary throttle plates open up, drill a series of four 0.030-inch holes, starting at 1/4 inch from the bottom and about 3/32 inch apart through the secondary fuel feed tubes. Caution: Be careful not to distort the orifices at the ends of these tubes.
If the intake manifold vacuum is extremely low at idle, the spring tension in the power valve or under the power piston may overcome the force of vacuum pulling the piston down (or holding the power valve shut). In an instance such as this, the power valve would open or the power piston would raise up, pulling the metering rods higher up in the main jets. This condition would cause the mixture to be way too rich during idle, off idle and even while cruising. A problem like this would not clear up until wide open throttle had been reached and the spark plug fouling from the over-rich mixture had been blown out. To overcome this problem, the power valve or power piston spring will have to be weakened or shortened. This can be done by clipping coils from the spring. Remove only 1/2 a coil at a time so as not to take off too much.
During acceleration, low manifold vacuum allows the power valve to open. Fuel flow through the power valve or past the raised metering rods effectively increases fuel discharge in the main well approximately 6 to 10 jet numbers during the time it's operating. In the case of the power piston/metering rod(s), all of the enrichment takes place in the main circuit, as the metering rod(s) is/are lifted up in the main jet(s).
The cooler the fuel mixture, the more power that can be made from that mixture, since it will be more dense.
Check to see if the manifold design will allow installation of a windage tray between the heads to keep hot oil from splashing against the bottom of the manifold.
Use insulator gaskets between the carburetor and manifold, if there is space.
Aluminum heat shields can be installed between the manifold and carburetor. These act as a heat sink and dissipate heat before it gets to the carburetor.
Route fuel lines away from the exhaust and the engine
Should be low restriction
Running without an air cleaner lets dirt into engine causing cylinder wear
Needs to get colder, denser outside air
Air cleaner base should be kept because its shape ensures an efficient entry path for the incoming air and helps to keep it from being heated by the engine
Flipping the lid may let slightly more air in - a tall open element air cleaner has the least restriction
Stack two air cleaners together if there is space
High performance air filter can result in 25% increase in airflow in a matching carburetor
No significant gain with a 500cfm filter on a 200cfm intake
Higher-density inlet air improves the engine's power capability in proportion to the density increase
Underhood air temperature is not ideal for horsepower production because the air has been heated by passing through the radiator and hot engine components
Gives more improvement than ram air because about 1% HP is gained for each 5°F decrease in temperature assuming the mixture has been adjusted to compensate and there is no detonation or other problems
Connect air scoops to a cold air box or the air cleaner housing rather than directly to the carburetor, to avoid creating turbulence in the incoming air as it enters the carburetor air horn. Retain the air cleaner element to diffuse the incoming air's turbulence, or a high speed miss may occur.
Duct cold air from the cowl just ahead of the windshield. It is a high pressure area that will supply cool outside air to the carburetor.
You can increase velocity of the air by gradually decreasing the cross-section area of the scoop or air box. However, this can cause turbulence.
Increased air density requires the use of larger main jets. Increase in size (area) is directly proportional to the square root of the density increase (percent).
Heated manifold riser
Reduces air density
Exhaust heat or jacket water to the manifold should be blocked off to create a cooler manifold when performance is being sought
Blocked heat risers are acceptable in summer, but need to be unblocked in colder temperatures to decrease engine warm-up time.
Route away from heat sources
Minor pressure increases
Helps when engine is running out of air at high RPM
Connect to intake box or air cleaner housing to avoid creating turbulence
May need to change air cleaner more often
Improves torque at one point or across a narrow rpm band
The longer the passage length, the lower the rpm at which peak torque will occur
Increasing the manifold-passage size, intake ports and valves raises the rpm where best filling occurs
Best driveability and street performance usually achieved with small port manifolds and heads
Must have 2" minimum clearance at top of stack
Adapted from How to tune, modify and repair Rochester carburetors by Doug Roe
The air an engine consumes enters through the carburetor, so knowing how much air an engine can effectively use will help us select the correct carburetor.
How big should a carburetor be? Two variables plugged into a simple formula can help determine the best size carburetor for an engine. They are:
Engine Displacement - specified in cubic inches. [Note: Displacements in cubic centimeters (cc) are divided by 16.4 to convert them to cubic inch displacement (CID) for the formulas.
Maximum rpm - the peak rpm the engine will be run.
You must be realistic with these values. Inflated figures will cause you to buy too large a carburetor, which is a waste of time and money.
Now, plug these numbers into the following formula:
((CID * rpm)/3456) * 1 = cfm
Example: 350 CID
7000 rpm maximum
assume volumetric efficiency of 1 (100%)
((350 * 7000)/3456) *1 = 709 cfm
A volumetric efficiency of 100% or 1 is usually not achieved with a naturally aspirated (unsupercharged) engine. So, unless supercharged, the example engine won't consume 709 cfm of air.
Volumetric efficiency (VE) - This is a measure of how well the engine breathes. The better the breathing ability - the higher the volumetric efficiency. Volumetric efficiency is an incorrect description of mass efficiency, which is the value actually being measured. But its usage is established, so the term is used here.
Volumetic efficiency is defined as the ratio of the actual mass (weight) of air taken into the engine, to the mass the engine displacement would theoretically consume if there were no losses. This ratio is expressed as a percentage. VE is low at idle and at low rpm because the engine is throttle by the throttle-blade position.
VE reaches a maximum at an engine speed close to where maximum torque at WOT occurs, then falls off as engine speed is increased to peak rpm. The VE curve closely follows the engine's torque curve.
Different engine types produce various VE percentages. Ordinary low-performance engines have a VE of about 75% at maximum speed; about 80% at maximum torque. A full-race engine has a Ve of about 90% at maximum speed; about 95% at maximum torque.
Air mass and density
The lower the pressure, the less dense the air is
A laminar-flow unit or other gas measuring devices such as a calibrated orifice or a pitot tube can measure actual mass flow into an engine while it is running.
Varajet (staged 2-barrel) Flow Ratings
2 barrel carburetors rated at 3.0-in.Hg pressure drop.
From the book: Tune, Rebuild, or Modify Rochester Carburetors. The author is Doug Roe. It is the 2nd edition
Fuel Flow Requirements
Fuel is consumed in proportion to the air being taken in
To convert airflow into lb per hour:
Cfm * 4.38 = airflow lb per hour
4.38 is a factor for 60F at one atmosphere --- 14.7psi
Multiply by the Fuel/Air Ratio, which we will assume to be a typical full-power ratio of 0.077 - A/F ratio of 13:1
Cfm * 4.38 * F/A *VE = Fuel flow lb per hour
VE = Volumetric Efficiency
Ideal fuel mixture: proportioned so that all the fuel burns with all the air, the exhaust has only carbon monoxide, water, and nitrogen.
Ideal F/A ratio is about 0.068
Ideal A/F ratio is about 14.7:1
Assuming ideal conditions, the actual ratio at which this occurs varies with the fuel's molecular structure.
Requires more fuel to combine with the air
Excess fuel required because distribution to cylinders and air/fuel mixing seldom perfect
Excess is usually 10-15% giving a F/A ratio or 0.075-0.080 or a 13.3:1-12.5:1 A/F ratio
Sometimes a F/A ratio larger than maximum power ratio used to internally cool an engine. Unburned hydrocarbons and carbon monoxide are undesirable products of the rich condition
Good maximum power mixture is 12:1 because horsepower will be strong and richer mixture aids combustion-chamber cooling to prevent detonation and preignition.
At about 7:1, the mixture no longer burns, and black smoke and extreme sluggishness occur.
Requires excess air to the engine.
If more air is admitted, so the A/F ratio is greater than 15:1, air will be left after combustion is complete, but most of the gasoline will be burned
The lean ratio could cause engine surge
In modern engines, if the A/F ratio approaches 20:1, the mixture is too lean to burn. Heavy surge, hesitation and backfire result
Little air and fuel enter the engine at idle
Because the pressure is extremely low in the cylinders, more exhaust gases remain in the combustion chamber
A 15:1 mixture can leave the carburetor, but be diluted by residual exhaust gases in the combustion chamber until it can be a lean as 20:1
The engine will run poorly or stall
Some fuel molecules combine with exhaust molecules and other fuel molecules align with oxygen in the air. The mixture is made 10-20% richer than stoichiometric to offset the fuel combining with exhaust. The richer mixture helps offset distribution problems, but it creates emission problems because rich idle mixtures generate excessive carbon monoxide.
Carbs built before 1980 supply a rich mixture in the range of 10:1 - 13:1 to overcome dilution.
Microprocessor-controlled systems introduced in 1980 often supply an A/F mixture around 14:1 - 15:1.
Beginning in the mid-70s, carburetor manufacturers narrowed the mixture requirements of specific engines. Then only minor adjustments were permitted - idle-limiter devices were put on the idle-mixture adjustment screw.
Current mixture adjustment screws are sealed with a plug and it takes some effort to get access to them.
Requires the richest mixture of all because slow cranking speeds create air velocity that is too low to pull fuel droplets
Both the fuel and manifold are cold, so vaporization is minimal. The fuel must be partially vaporized to get to the cylinders in a burnable condition
Once the engine fires, speed goes up, velocity through the carburetor improves and vacuum increases. The result is better vaporization and the liquid fuel deposited on the manifold walls vaporizes. As the engine warms up and the choke comes off, the mixture is leaned out to normal.
Effects of Inlet system changes
High Float level
Raises fuel level in bowl. Speeds up main system start-up because less depression--reduction of pressure--is required at venturi to extract fuel from bowl. Start of flow from the bowl via the nozzle is sometimes called pullover. Increases fuel consumption. May cause fuel to spill through discharge nozzles and vent into carburetor air inlet on abrupt stops or turns to cause over-rich air/fuel mixture. Engine then runs erratically or stalls. This spillage affects low rpm emissions. Increases chances for the carburetor to percolate and boil over. This is a condition in which fuel is pushed by rising vapor bubbled out of the discharge from the main well when it is hot. Also, when the vehicle is parked on a hill or side slope, high float level may cause fuel spillage through the vent or main system.
Low float level
Lowers fuel level in bowl. Delays main system start-up because more vacuum is required at venturi to start fuel flow. This delay may cause flat spots or holes in power output. May expose main jets in hard maneuvering, causing turn cutout--misfire from lean air/fuel mixture. Can lessen maximum fuel flow at wide-open throttle--WOT.
Float assist spring too strong
Causes low fuel level. Inlet valve closes prematurely because of added force of spring.
Assist spring too weak
Causes high fuel level. Inlet valve closes after correct fuel level is reached because more float movement is required to compensate for weak spring.
High fuel pressure
Raises fuel level approximately 0.020-inch for each psi--pounds per square inch--fuel pressure increase. This factor varies with bouyancy, leverage, and needle-orifice size.
Low fuel pressure
Lowers fuel level.
Larger inlet-valve seat
Raises fuel level.
Lowers fuel level.
NOTE: Changing almost any part in fuel-inlet system requires resetting float to maintain correct fuel level.
This chart is from the book: Tune, Rebuild, or Modify Rochester Carburettors. The author is Doug Roe. It is the 2nd edition.