carb question

[john tynan]

I had my fulvia running lovely. No problems ticking over. I had set up the balance of the solexes perfectly with vac gauges, and used a glass plug to look at the combustion for the idle adjust mixture for each cylinder.Then last week it started to run lumpy. Number 3 cylinder was not getting fuel at idle, but was ok once you revved it. I stripped the carb and cleaned all the jets and ports. blew out everything with compressed air, and put it back in. Still no juice at idle. There's plenty of vacuum at the throttle body and I checked with a gauge and it's the same at each of the 2 trumpets for that carb. The spark is great and it pulls great from around 3k revs. I'm stumped. I just won a seal kit on e-bay, so I'll rebuild both carbs when the kits arrive, but i'm worried that there is something else i'm missing. Any thoughts?

[tim]

There are some buried set screw jets slightly behind the mix adjuster which sometimes are too finely set and these can get the required mixture through. You can see some depressions and underneath the "wax" is another "factory set" screw. My No 3 had the same problem!

Tim

[Huib]

These bypass screws are tricky. A four column vacuum meter connected to each of the barrels is needed. Also it has to be sure that the cilinders are equal and the carbs ok so that the bypass screws are used only to compensate for the difference in opening of the butterfly valves and not for other problems.

[john tynan]

I am getting no combustion in no.3 cylinder at idle. there is plenty of fuel igniting once the throttle is opened.I connected compressed air to the chamber that leads to the aperture which the idle needle sits into, and there is no blockage. Would I be correct in thinking that if the valve clearances were too tight on the exhaust side of that cylinder, then at low revs, the fuel would evacuate on the compression stroke, but at higher revs, you would get some latent fuel in the cylinder?

[P. de R. Leclercq]

I know you have cleaned everything etc., etc. but it still sounds like a blocked idle jet to me.

Try swapping the idle jet from No 3 with one of the others and see what happens. It's only five minutes after all!

Paul

[john tynan]

tried swapping jets and needles with no change......it's driving me mad, because I could drive it and keep the revs up, but I won't in case I run too lean a mixture and hole the piston. I don't have the time to mess with it until the weekend.

[P. de R. Leclercq]

You would only have such lean mixture were the mani jet(s) obstructed.

The problem could also be related to the drillings in the carb bodies - you can see little holes ("progression holes") near the bottom of the butterflies.

Paul

[john tynan]

still no further with this problem. waiting on carb kit from e-bay.....
Valve timing and clearances checked and ok. swapped floats and needles from another carb, but still no joy. the passage from the jet to the idle needle is clear, so next job is to whip out the carbs again and blow back into the float chamber. haven't driven the car in 3 weeks now. depression imminent.

[Huib]

You are right about worrying about burning holes in a piston as a result of a lean mixture. However, I don't think this is going to happen at idle. There simply is not enough energy going around to ruin the engine. Neverteless the problem should be solved, even if only for peace of mind. And don't use high power just to make sure nothing bad will happen.

Any difference in colour of the plugs after a reasonable length drive?

What plugs do you use?

You did not say what type of Fulvia nor what type of carb

Did you check the fuel pressure?

Is the support under the filter housing still ok? It should be firmly attached to the oil filter base and to the air filter box. No movement at all.

Did you hold each carb against a light source and compare the amount of light shining around the valves with the idle screw fully retracted?

From your postings I understand that you pressed a vacuum meter against the trumpets. I am not sure how much information that gives about air leaks around the valves or via the flanges of the carb rubber. I use a 4 column vacuum meter after the valves. Dellorto's have a vacuum connection on each barrel. For Solexes you have to make M5 holes in 1 and 4 on the manifold.

Any difference in compression?

[tim]

I am sure you need to investigate the secondary idle screws I mentioned earlier. BTW stripping the carbs is not straightforward whe it comes to resetting the butterflys as a slight difference in closing in each pair can screw the idle stting - maybe this is the problem if you have already touched them.

Tim