Weber 40 DCOE fuel/petrol leak

[hmeleg]

Can anybody help me with this one as other posts have not helped so....

I have a twin 40 DCOE setup on a Ford Lotus Twincam and even when idling see fuel leaking back out of choke/ram trumpet on pot 3. Others are dry and carbs have been rebuilt recently but still same problem. Have taken great care to ensure that float settings are correct (but if float was wrong then wouldn't it leak out of both 3 & 4 ?). Also butterflies on 3 & 4 appear to be the same and open/close together so no difference there. When idling and throttle link disconnected, vacuum reading on 3 is maybe 0.5 greater (scale of 1-5) than on 1,2 or 4 across the ram intakes. And if I use a infra red temp gauge on exhaust then this No.3 pot seems to run 50-70 deg C hotter than others. This seems to indicate weak running on this pot but why would that cause a fuel leak when vacuum is slightly higher. Wouldn't that tend to suck and excess fuel in ?. All the jets across both carbs are identical and idle screws are new and turned the same amount out (approx 1.5 turns). Anybody out there who can help Mr. Baffled here ?.

Thanks in advance.

[Justin S]

How flexible are the mounts/ o ring/ misab plates to the inlet manifold, as this can cause issues as well.

[hmeleg]

I use proper O ring mounts. These are the aluminium sandwich types with separate O rings on both faces. The carb/manifold bolts are capped with springs underneath and I have tightened them up evenly to allow the carbs to rock slightly when pushed/pulled up/down. I believe that tightening them up solid is a bad thing that would allow vibration to froth the fuel up in the float chambers. Now following this train of thought, if one of the mating faces on barrel 3 was distorted and didn't seal fully. this might let in air between the carb and the cylinder. This would account for it running weak and hot. But would this allow petrol to leak back out of the trumpet ?.
Any suggestions for testing for a leak whilst the engine was running ?.

Thanks.

[blitzracing]

Any idea what you current pump pressure is- after all it would only take a very small increase to push the float down against the fuel in the bowl. On the off road buggies I used to mess around with we always had to put a variable pressure regulator in line with the carbs to stop them leaking across rough terain. It was a delicate ballance.

[hmeleg]

But if this was the issue would it not cause a simultaneous leak on both pots 3 & 4 which share the share carb ?.

I was thinking of maybe trying to temporary wrap that individual flange on pot 3 with silicone sealant or ptfe tape and after letting it dry, try starting the engine. If the leak disappears then I could peel the stuff off and see if it starts leaking again. That would prove a air leak on the flange (is that is causing it !).

[hmeleg]

Can anybody out there confirm that if I had an air leak on the manifold it would cause petrol to leak back out of the intake on that barrel ?.

Thanks in advance.

[blitzracing]

You are definately getting the leak at idle only? You can get a situation on some engines setup called reversion, where the fuel air mix gets thrown back into the mouth of the carb or even the air filter, that will occurr a specific RPM bands. Usually associated with high perfomance cams and short inlet tracts.

[hmeleg]

Interesting - that would probably apply here. The cams are .400 lift which is a fair amount for one of these engines and very short trumpets. Think I will explore for air leaks first given that No.3 is running hot and see if I can confirm this with my Colourtune. After that may try to obtain some long trumpets and try that too.

Thanks for the advice. Will post up the results here later for other folks.

Regards.

[TonyBrooks]

It may not be connected but I had a similar problem on a completly rebuilt set of 40 DCOE's in that I had fuel leaking out of the ram pipes on no 1 & 2 cylinders. When I took it to a rolling road the guy there always strips a carb down so he can make sure that all is right before he starts adjusting jets etc. He discovered that my main jets on the front carb were from a different weber carb and allowed the fuel to come out at 90 degrees instead of straight down (or the other way round if you follow me) - putting the rights jets in solved the problem. Just shows you can't always trust a so called expert carb rebuilder.

[blitzracing]

You might see the reversion if you remove the air filter, and look across the face of the trumpets, and then rev the car quite high, it may be seen as a mist of fuel going the wrong way out of the trumpet, and if its bad the engine will run lean and splutter.