Edited by T from original submissions by John (Arsewipe) November 27th 2006;
Some additional info and images by named contributors:
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What is Vapourisation:When the Engine temperature soars, Fuel on its way to the Carburettor can turn into Vapour.
The amount of Vapour can exceed the Float's ability to differentiate between it and Liquid Fuel in time to keep the supply of liquid Fuel to the Engine. Vapourisation Thread
Technically, Vapour Lock is something that can only happen to a Fuel Injected Engine.
Early Fuel Injection Systems made no provision for the presence of air and were incapable of differentiating between liquid and vapour in their lines. Vapour Lock is still a problem in Fuel Injected Aero Engines and can be induced simply by closing the Throttle too rapidly.
Modern Fuel Injection Systems place the Fuel Regulator at a noticeably high point so that
any Air or Vapour in the Fuel Rail will be returned to the Fuel Tank before any Liquid.
Submitted by lovemyholden on Wed, 11/04/2007 - 11:42.
When I recently did my V8 conversion, I had vapour lock problems. The engine would run fine, even idle for 10 mins no worries, but as soon as I shut it off, that was it. No restart possible for 10 mins. Opening the bonnet would reduce the recovery time to 5 minutes or so. I traced the fuel line (metal) and it ran within a couple of inches of the Right bank manifold. I decided to bypass the offending section of pipe completely and now all is perfect, no more problems.
It's an important point, that this never caused my engine to cut out or faulter, but would render it unstartable for a period of time after stopping it. My reasoning is the heat soak factor after the engine stopped was enough to 'cross the line' so to speak. I snuck a look under the bonnet while the engine was idling, and the filter had fuel in it.
However once the motor had stopped for a minute the fuel filter would empty out of fuel, so I just knew what was going on. My theory is that even at idle the car was consuming enough fuel to be opening the needle and seat and purging the fuel through, in other words keeping the fuel moving. Also worth noting is I run an electric pump.
Another interesting point is the routing of the fuel line. I checked a factory V8 Van and noticed the metal fuel line was mounted more on top of the chassis rail, moving it away from the manifold at least another inch. My fuel line was still in it's plastic clips so I don't think someone moved it at some time.
This begs the question did the factory route the engine bay fuel lines differently for 6 and 8's. If so this may explain why some cars suffer from this and others don't.
Cheers 'T' and all
End of submission by LMH
Causes and Cures:
Submission by John (Arsewipe):
The T Piece prevents fuel boiling and causing vapour problems.
Better with it closer to the carby, (the Weber looks as though it has a return from the Carby anyway) and needs to be smaller than the main fuel line to prevent the fuel just recirculating and not going into the carb.
This was seen a lot on the Hemi's as they ran quite hot.
Submission by T Apr 13th 2007:
The hot start issue is related to the fuel boiling in the carburettor after the engine has been shutdown.
Many carburettors provide space for fuel expansion under these circumstances but problems can arise
when the needle and seat have faces that are other than steel/brass.
Because the rubber type can become soft with temperature, the float level rises during the run time
of the engine permitting the float level to rise.
When the engine is shutdown and the carburettor temperature rises the fuel expansion space in the float
chamber is no longer available resulting in fuel spilling into the inlet manifold, vapourising and
saturating the air cleaner in fuel and vapour.
The result is a very unco-operative hot start.
The rubber needle and seat also results in a flooding graveyard spiral when the fuel in the line from the
fuel pump enters the float chamber after the engine has stopped.
Replacing the needle and seat type and lowering the float level results in a faster flow rate of fuel
entering the carburettor and more expansion space.
Adding a thermal insulator between the carburettor and inlet manifold can keep the carburettor temperature down.
Covering the fuel lines with split rubber fuel hose minimises the temperature in the fuel lines.
Covering the fuel lines with heat reflective material minimises the temperature in the fuel lines as well.
Rerouting fuel lines away from heat sources like manifolds helps too.
End of submission by T.
cheers Dom. Why is the return line used? The reason Ford Australia added it to every Weber equiped Pinto engined four cylinder since 1973, and every Weber equiped six cylinder from mid 1982 was because of 1. fuel vapourisation (hot fuel handling problems),
2. fuel leaks (Webers leak fuel above 5 psi),
3. hydrocarbon emissions,
4. fuel fires,
5. and the quest for better fuel economy (lower needle and seat pressures reduce the over supply of fuel on bumpy or hard cornering conditions).
6. Before the advent of return lines, all Cortinas and Falcons had chronic fuel surge problems around corners because of fuel starvation. Weber AD carbs with return lines are just brilliant in that situation, because they were orginally used in the French Simca 180's with soft suspension, and the Weber AD series used in these were resistant to fuel surge. Eliminate the return line, and all six items become issues. Even ripping off the evaporative emissions increases green house gas emission by 20%! In the XE to XF engines, the Weber ADM 34 recieves 5 to 6 pounds of pump pressure, just like the early ones.
What is different, is the carb bleads off the other 2 to 2.5 pounds of pump pressure by virtue of a 0.5 mm bleed line out of the needle and seat valve.
So the carb only sees 2.5 to 3 psi tops in operation.