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Page created by Jacks August 15th 2006:

Back to the Engine Section

Note the Rotor and Harmonic Balancer positions. The Rotor is pointing to the timing mark on the Distributor. The Harmonic Balancer marks line up. Note both Valves on Cylinder 1 are closed. Note that the Distribuotr is one tooth position too far anticlockwise. The centreline of the Vacuum Advance unit should be parallel to the centreline of the Engine. Click to Enlargen. Photo by MICKHG1970.


It is from the Distributor that the ignition Timing is set, the setting in turn is controlled by the relationship of the position of the piston to when the spark occurs.
If the ignition spark is not set correctly then you have combustion at the wrong time. This will cause pinging etc, and will also cause some backfires out of the carby. A loss of power and high combustion temperatures, wasted fuel, higher emissions and of coarse damage to the internals of the engine can also occur, so it's important to get the Distributor timing correctly set.
This image shows the correct position of the Distributor Body, with the Vacuum Advance Module centreline parallel to the Engine's Centreline. Note that the crankshaft is not at the Cylinder 1 TDC position which is why the Rotor Button is pointing away from the TDC Timing Mark.  Click to Enlargen. Photo by Phillip Gard

You can control the distributor timing, providing you know what and how the working of the distributor operate.
There are three component in the operation of the Timing associated with the Distributor, namely
Initial Timing, Centrifugal Advance and Vacuum Advance , so in essence the distributor can have up to three timing setting, sort of.
As your engine revs increase then the time required to fire the cylinder is diminishing, as parts are moving faster. So, you must get the spark in there sooner, but not too soon that it will cause detonation or engine damage as advised earlier.

Now in order to change the ignition timing, or the ignition curve, one really needs to know a little bit about the workings of the distributor.

==Initial Timing:==

First of all we have the Initial Timing, this is the mark that you have set your timing up in relation to the timing mark on the front pulley, know as the (Harmonic Balancer) some engines have their timing mark on the Flywheel

Here is how it works on most Oldholden owner engines, some would have either simple mechanical advance, or a combination of Centrifugal and Vacuum advance system built into the distributor. Some might even have a little bit more up market, using a HEI distributors. These will give a bigger and better spark across the full rev rang, as there are no breaker points in this type of distributor, so they're ideal for LPG
>[Bosch HEI]
First off ,you need to set the " Initial Timing�? using a timing light, setting it at the balancer mark /tab indicator, or fly wheel mark.

So I’m assuming you know a little about how to get Top Dead Centre,(TDC) in order to get the timing mark lined up with your number one lead point marking on the distributor ridge, if not then look here (link for TDC)

In saying that, I might add that when setting your initial timing by the engine specification, from TDC one must bear in mind that your engine is stock and in very good condition, if you have wear in the timing mechanism leading up to the distributor, such as the centre hub of the harmonic balance could have moved around, or bad cam lobes etc, then the timing mark will not be correctly in line with the manufactures mark.
(link harmonic balance)

However, if you can connect a vacuum gauge up, then by moving the distributor (at idle speed) so as to get the highest reading on the vacuum gauge, at that point your will be just about spot on for the Initial Timing mark, so you can then mark that NEW position and move on from there as to what timing you after.

Further details on vacuum tuning are here
Vacuum Tuning

Centrifugal Advance

Centrifugal advance or sometimes called mechanical advance. It is governed by the speed of the engine (RPM)
Inside the distributor housing is an advance mechanism that is integral with the distributor shaft, it consists of two springs and a pair of advance weights situated on the cam weight base plate, also an advance cam that’s integral with the distributor cam.

At low speeds, the two springs hold the pair of advance weights in, so there is no additional spark advance. The sparks occur only in accordance with the "initial manual" setting of the your distributor setting.
When you increase the revs, the Centrifugal/mechanical advance made by the weights and springs situated under the breaker base plate in the distributor housing come into play. Advanced timing occurs as the springs that hold the weights are now pulling in the opposite resistance to the force that controls the speed, or the rate, these weights are moving outward, caused by the centrifugal force. This in turn is causing the distributor cam to move on the shaft, so it in turn now "advances" the timing more.
So as speed increases, the weights move outward moving the advance cam ,thus rotating the distributor cam ahead of the distributor shaft, causing the distributor cam lobes to then open the contact points earlier in the compression stroke so that the spark is advanced,
click to enlarge Centrifugal Advance Slightly Open

click to enlarge Centrifugal Advance Fully Open

As the centrifugal forces take over thus pushing the advance plate mechanism even further until it reaches its stop, then you have Total timing of the Distributor as no further advancing of the distributor can take place.
Thereby changing the timing causing the point that the spark plug receives, also at this point so the voltage to will also change in relation to the piston location.

To check if your the Centrifugal Advance assembly is working, remove your distributor cap, then rotate the Rotor button. It should rotate slightly, and you will notice it moves the cam lobe on the shaft slightly, advancing the timing. When you release the rotor the weight springs will pull it back into the "retard" position,
This movement being slight should also be quite smooth in its operation. If it's sticking or not working then it needs to be overhauled and fixed
The main reason for failure is lack of lubricant, as the working of the Centrifugal Advance assembly receives lubricant via a felt Pad situated under the Rotor button and located in the distributor shaft. This Pad needs a few drops of oil from time to time. It's often over looked, and some people don't even know it's there.

Changing Mechanical Advance:

As you can change your initial timing by changing the position of the distributor, so it is that you can also make changes on the mechanical side of the timing as well, in order to get the best possible performance from your engine.
The rate of advance can be done by changing the weights or springs, as the mass of the weights and the springs' pressure will determine how quickly the curve reaches its maximum advance.
You can also increase the advance by lengthening the slots on the advance plate.
If you need to reduce the total advance, brazing up the slot will do that.
This modification to your distributor is commonly know as re-graphing the Timing Curve.

==Vacuum Advance==

click to enlarge Breaker Plate

Mounted on the side of the distributor body is a Vacuum canister module. Inside the canister is a spring loaded diaphragm, and prodding from this is a flat pull rod that is linked to the advance Breaker plate inside the distributor housing.
When the canister receives a vacuum signal from the engine, it in turn operates the diaphragm, and the flat pull rod that’s attached to the Breaker plate securing the contact points. It then rotates around the distributor cam, advancing the timing
As the spring loaded diaphgram is operated by the throttle, it’s imperative that the vacuum be received from above the butterfly, (atmospheric side).
The reason being is that when the throttle is at idle position then there is no vacuum to operate the Vacuum advance. This vacuum is called "Ported Vacuum" and there is also "no load" here on the engine.
However, when the throttle is open, the vacuum signal will be sufficient to operate the control diaphragm causing it to compress the spring and pull the breaker advance plate to rotate in a counter clockwise direction thus moving the contact points so distributor cam lobes open the points earlier in the compression stroke.
The movement in the throttle opening determines, in part, the amount of intake manifold vacuum and thus this then determines the amount of spark advance obtained.

This means that Manifold pressure during acceleration or pulling with wide open throttle, will not be strong enough to hold the breaker plate in the fully advanced position, so then the spring loaded diaphragm will then release its pressure allowing for the advance plate to return back to the fully/partly retarded position, depending upon the manifold vacuum available.
The advance obtained by the vacuum control is added to the Centrifugal/Mechanical advance

NOTE: There are some important things here to bear in mind, your engine needs to be in good condition, as well as carburetor and timing being correctly set, the engine should then be giving around 20�? vacuum , as it is at idle when the highest vacuum is sort.
Make sure there are no leaks in the vacuum system. Very important and often over looked, check the vacuum hose at each end as that’s where they normally fail.
In the mechanical advance, the timing is done by the engine RPM, however in the vacuum advance the timing is instantaneous, as it senses the changes in the engine load at idle.
However if your vehicle is automatic then vacuum is not ported, use manifold vacuum for the vacuum advance.

Two Types of Vacuum

Ported Vacuum
The vacuum signal that the Vacuum canister receives is related to the engine RPM called Ported Vacuum. It’s received from Above the butterfly of your carburetor. As vacuum increases to its maximum, so too does the pull on the rod pulling the advance plate advancing the timing, as this is required for the light load at idle,

Manifold Vacuum
It's different as its there all the time, while engine is running. It’s not RPM related (used for vacuum brakes booster)

Also while on RPM, it’s important to note that as the RPM increase the timing position for the firing of the spark inside the cylinder changes. This rapid movement is measured in milliseconds, so it’s important that the timing and firing be at optimum level.
As the distributor can only vary the spark timing in proportion to speed, then it’s here that the vacuum advance will give it a far greater degree of advance range. That is what’s required at idle and cruise conditions as these are normally associated with light load "vacuum advance".


So as you can see there are benefits in having vacuum advance. It improves your idle along with cooling, the throttle response is smoother so helps in the economy, along with maybe allowing for a leaner fuel jetting at light load. Will also help with spark knock when under full throttle.

The rod movement of the canister can be modified if need be, and can either be changed in the strength of the pull or the distance the rod travels.
The longer the travel/pull, the more vacuum advance will be applied. The lighter the spring the easier it will be for vacuum advance to be applied and the earlier the vacuum advance will reach its maximum.
Vacuum Mods
Photo by Jacks showing a close up of the Vacuum Advance Module. Note the actuating hole in the pull rod/arm and the number 212 stamped on it.

In this picture you can see the stamping 212, this means 2 units of power and the rod travels 1.2 mm in length.(from HEI Distributor)
First number is the strength of the pull (units), the other is the distance the rod travels(mm),
At the end of the rod is a hole, this locates onto vacuum plate, as the rod is pulled it advances the Timing, till it reaches the stop.


Vacuum advance units are sensitive devices. They only operate on vacuum which needs to be strong enough to achieve the maximum advance. The greater the vacuum the more advance. At idle, the throttle is almost completely closed so that is when vacuum is at its highest, so by tapping the vacuum for the advance system above the butterfly we can control the vacuum with RPM.
Too much vacuum advance can result in part throttle ping and poor economy. Insufficient vacuum advance results in poor part-throttle response as well as poor economy, so it would definitely pay to get your distributor regraphed to suite your engine needs. It is a very slow and complex bit of work to try and do yourself without the right gear.

The way I see it, vacuum advance gives a smoother flow in the early advance of ignition timing, so giving a smoother take off. As the vacuum is smooth in its operation as well as being instantaneous, it sort of irons out the lumpy bits that would normally be present had there been no vacuum advance fitted. There is no doubt you would probably need to get a bigger squirt of fuel in the carby as well with your accelerator pump to compensate for getting the vehicle moving. The sudden aggressive shift in RPM would be noticed, and you would also lose some valuable fuel as well, so the vacuum advance movement sorts out the lumpy bits somewhat if you get my drift. Then, the next stage in the distributor timing comes into play, as the RPM for the mechanical advance is now reached so it can now carry on the smooth flow of the RPM.

There are some schools of thought that say, as we can no longer get Leaded fuel, then maybe it might be better if the vacuum be switched from "ported vacuum" above the butterfly to the "full manifold port". As higher Octane fuel is being used with a ported vacuum set up, it can cause detonation problems, over heating, and other grief. Still, that's a personal choice, and nevertheless it seems to make sense.
You could connect straight manifold vacuum to the vacuum canister but it will add more ignition timing at idle, but if you have low manifold pressure and a big camshaft then it might be ok to experiment there.
However, if your're running a higher Octane fuel such as LPG then maybe this may be the way to go as there are many thoughts on this.
In my LPG set up, my Initial /Static timing (HEI) is on 12 deg advance, but I do have the vacuum canister disconnected when on LPG. When I Switch back to Petrol then I change to 6 deg advance and re-apply the vacuum advance canister again. That doesn't mean that that's the best way to go, it's just something i'm experimenting with as I can change my static timing as I drive, ok.

Total Timing:

It is the combination of the initial timing, plus mechanical timing
Example: say initial timing is 8 degree advance, and mechanical timing is 24 degree advance, the combined combination adds up to 32 so the Total Timing is 32 degree advance.

The springs inside the distributor housing can be changed either one or both. By making them lighter it will make the advance occur faster, a heavier stronger spring will slow down the advance rate. You can file the stop weights as well depending on what you need to achieve in the in the Timing Curve.
You can disconnect the vacuum line as I have (vacuum solenoid switch) when on LPG as this reduces the Total Timing, as my Initial timing has already been advanced for the higher Octane that LPG has.
However that's not the best way to set it up. I really need to regraph my dizzy if i'm going for straight LPG, or a dual curve ignition if staying on duel fuel.

Finding No 1 Spark

Setting ignition timing without a timing light by hand

12 V DC Timing Light. Image by Jacks.

Here is a way you can set you ignition timing points without the aid of timing light. You need to turn over your engine by hand.

You will need (see picture) a small trouble light that you can easily make yourself, simple DIY job, also a handy little devise that can also be used for checking other electrical circuits with as well, a must for your tool box.
Before we start, it is assumed the you have correctly set the cam angle on the points as specified for your distributor,

Remove the distributor cap, make sure the ignition is off, clean the timing “nick�? on the harmonic balancer as well as the timing gear cover pad, you might even highlight the nick with a white marker as well.
Now rotate the crank by hand so that the “nick�? on the harmonic balancer is lined up with the longest timing mark on the timing cover pad, this must be done with the No 1 piston on the compression stroke.
Some Tips, the more plugs removed the easier it is to turn the crank by hand, and always turn the crankshaft in the one direction only (clockwise) don't come back if you have over shot the timing mark, as your timing will be out of whack, reason being is that there is wear in the cam timing chain etc (backlash).
You will need a 12v lead light, so if you have one all well and good if not then it’s a simple DIY job. Here is a pic of mine, have had it for ages.
Anybody with an Injected car will find this an invaluable tool to have as well, as it can also be used for testing the presence of injector pulses.

Ignition Settings Chart:

Distributor Ignition setting (6cyl)
Model Initial Degree Manual - Automatic Centrifugal Maximum rpm Vacuum Maximum
1948/53 7° b.t.d.c. 22° at 2500 18°
FJ 7° b.t.d.c. 22° at 2500 18°
FE 2° b.t.d.c. 28°- 32° at 3600 18° at 13” Hg
FC 2° b.t.d.c. 28°- 32° at 3600 18° at 13” Hg
FB 2° b.t.d.c. 22°- 26° at 3600 18° at 13” Hg
EK 2° b.t.d.c. 6° b.t.d.c. 22°- 26° at 3600 18° at 13” Hg
EJ 2° b.t.d.c. 6° b.t.d.c. 22°- 26° at 3600 18° at 13” Hg
EH 5° b.t.d.c. 5° b.t.d.c. 38° at 3600rpm 20° at 9" Hg
HD 5° b.t.d.c. 5° b.t.d.c. 38° at 3600rpm 20° at 9" Hg
HR 7° b.t.d.c. -7° b.t.d.c.  33° at 3300rpm
Hk 6°-8° b.t.d.c. 6°-8° b.t.d.c 33° at 3300rpm 11°-15°at 9" Hg
HT 6°-8° b.t.d.c. 6°-8° b.t.d.c 33° at 3300rpm 11°-15°at 9" Hg
HG 6°-8° b.t.d.c. 6°-8° b.t.d.c 33° at 3300rpm 11°-15°at 9" Hg  

Contact Points Gap:


1948/53 : .018" - .020"

FJ : .016" - .020"

FE,FC,FB,EK,EJ,EH,HD,HR,SEREIS : .012" - .016"

HD : Lucas .014" - .016"

HK,HT,HG, SERIES: Bosh .019" - .025"

HK,HT,HG, SERIES: Lucas .017" - .024"

If you want to add to this please do, if any mistakes, please rectify,
Cheers, Jacks 




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