Problem with #2 at idle

[sprintsoft]

"1.5 kg/h on the Syncrometer vs. 4.5 kg/h."
this shows the problem is flow related, not ignition..

and jets have already been swapped with the trial carb swap...

[LaikaTheDog]

Did you reuse the misab Ali/rubber gaskets when you swapped the carbs around? I chased a similar issue until I just got new ones, one of the old ones had a flatspot.

[CG901]

Thank you for all the helpful suggestions. Yes, we checked for leaks at the rubber O-rings and replaced plugs. No change.
We have not checked the cap for cracks. Will have a look today.

[HCA]

I did not twig the vac readings of your first post, so do not think it is anything down there.

Does the exhaust temperature of #2 match the others at higher revs?

[CG901]

Exhaust temp in #2 exhaust reads 265-280F vs 460-475F on others at high revs.

[HCA]

I cannot think of any one thing that would answer each issue, but there must be something!

[elans3]

I had a similar issue some years ago in my second S3. Took me ages to find the cause. Turned out to be a faulty headlight vacuum non-return valve, the one that screws into the inlet manifold.. Fitted a Norgren one and solved the problem immediately.
Remove it, and if you can blow through it from both ends, it's faulty.

[sprintsoft]

Hi CG,

looking back I notice you had the exact same issue on Nr 4 cylinder a while back, was that ever resolved?

Is it possible the carbs were swapped front to back, so now the same problem has been transferred to Nr 2?

Iain

[Quart Meg Miles]

I had a similar issue some years ago in my second S3. Took me ages to find the cause. Turned out to be a faulty headlight vacuum non-return valve, the one that screws into the inlet manifold.. Fitted a Norgren one and solved the problem immediately.
Remove it, and if you can blow through it from both ends, it's faulty.

That's on cylinder 1.

As the temperature is low on #2 at high revs it's not just an idle problem and as it has high compression like the others it's not a leaking or mis-timed valve.

A blocked exhaust port seems highly unlikely but could be checked by comparing cylinders when blowing down the spark plug hole (using a bit of heater hose?) with the exhaust valves open (one at a time, of course).

Leakage at the inlet manifold is still the obvious cause but should have been discovered when Webers were swapped. Is there a crack in the manifold casting or the rubber ring holder? Were the two Webers rebuilt incorrectly at the same time e.g. have the starter jets been left out or misplaced on that side?

"A healthy spark" suggests the dist cap is OK but is the suppressor cap high resistance? I'm not sure that would give the symptoms but is simpler that some other checks.

[2cams70]

When you say you checked the valve clearances did you check on number 1 that the shim was correctly contacting the top of the valve stem and not the spring retainer? Unlikely but check the cam lobes on that cylinder too. Was the head ported during rebuild? Check that there has not been any breakthrough anywhere in the casting.

[elans3]

I had a similar issue some years ago in my second S3. Took me ages to find the cause. Turned out to be a faulty headlight vacuum non-return valve, the one that screws into the inlet manifold.. Fitted a Norgren one and solved the problem immediately.
Remove it, and if you can blow through it from both ends, it's faulty.

That's on cylinder 1.

Not so, see this thread which I started at the time. viewtopic.php?f=19&t=1346&start=30
Page 3 gives the solution, after great responses from all our Forum experts, the vast majority of which were tried. The miss would change from cyl 1 to 2 and then 4, in a random order.. Don't ask me why but it danced around the cylinders.
I had a similar issue with an Alfa Twincam servo non-return valve that screwed into the inlet manifold, a few years later. Changed that and all was well. Never had it since.
Would suggest that for the time it takes to check, don't discount it.

[CG901]

Thank you. That is worth a try. Mine non-return valve, as I recall is located near the T- connector on the chassis cross member. I changed it a few years ago. Should it be mounted near the manifold? I would think farther from a heat source woud be best.

[elans3]

Under normal circumstances on a T/C, they are screwed onto an adaptor, which screws directly into the inlet manifold adjacent to number 1 cylinder. However, regardless of where the N/R valve is placed, if it's faulty, it will have the same effect.

[Pfreen]

A defective brake servo (if your car has one) can have the same effect and allow unmetered air to enter the intake manifold.

[elans3]

Correct, especially if it has a defective n/r valve