Help needed diagnosing Weber 40 IDA 3C problem

[1QuickS]

Your idle jet carriers are correct for your carbs, o-rings not really important, original design of carbs did not use o-rings, I suspect they were added to help keep fuel from dribbling down the side of the carb body, nothing else.

Check idle air correction jets on top of carb body and check emulsion tubes for cleanliness of holes along their length.

[edmayo]

Since you say the car ran fine for for 300 miles with all the same parts that are on it now lets assume that the pieces are basically okay, now in between times it was worked on, do you know if they checked and/ or adjusted timing? Setting ignition timing on a 14/6 is not easy, possible to make a mistake. It sort of sounds like maybe the timing is retarded,,,,just a possibility. Might ask if they did check or adjust timing.

[1QuickS]

So, engine runs fine to redline during WOT operation but at less than 75% throttle it misses. How does this suggest an ignition/timing issue?

[edmayo]

If the timing were set to the wrong marks on the flywheel, which is how you're supposed to set timing on a 914/6, where you really can't use the front pulley, it can happen. If timing is retarded power will be off until enough advance occurs to pull better, even though the total is still off. It just seemed strange that it ran fine until sometime after it had been worked on. It would be just one more test to start eliminating possibilities. If its good then we move on. I try to start with the easy and obvious stuff first. One of the first checks I tend to do is to pump out the gas tank and check that oft forgotten sock filter. Usually I'm glad I did!

[Bob Schaefer]

So, engine runs fine to redline during WOT operation but at less than 75% throttle it misses. How does this suggest an ignition/timing issue?

My experience with this problem indicates an excellent correlation between throttle position / rpm and the missing. It’s not random.

And don’t rich mixtures at high rpm tax ignition system strength more than lower rpm, less rich mixtures?

[Bob Schaefer]

Ed - Not being aware of the Weber idle circuit / idle jet debris sensitivities a year ago when I first encountered missing, I didn’t check WOT operation to redline because of driving conditions immediately before noticing the missing. As I speculated previously, I may have mis-diagnosed the problem when I changed plugs the 1st time. It’s a mystery why operation went back to normal afterwards, only to begin missing again about 100 miles and 11 months later. Timing was “checked and verified” by the Porsche shop before I took delivery, and not touched since.

What’s the explanation for the coming & going of the missing somehow coinciding with mysterious timing changes that would have to have gone from good to bad, back to good, and finally back to bad if there’s a correlation between them?

[edmayo]

So to review, the carburetors are set, jets cleaned and float levels correct, fuel pressure is correct, fuel pump is good since it will pull to redline, spark plugs have been changed and are correct type, Ignition timing is verified correct, cap and rotor checked for arcing, what ever type ignition system is on the car is working fine since engine will pull redline, the problem is infinitely repeatable strictly due to throttle position, except for the times it runs perfectly. Have I missed anything? A real stumper.

[1QuickS]

I remain suspicious of the Webers having ingested a bit of debris on either one or more idle air correction jets or on an emulsion tube. Partial throttle would demand fuel delivery through these components but WOT is main circuit only. I have personally experienced these two scenarios, a bit of rust from air cleaner blocked an idle air correction jet and another time a bit of paper towel blocked a small section of one emulsion tube and both times I had WOT to redline but troubled partial throttle operation.

Another item to ponder is fuel levels may become erratic either from the needle valves being side-loaded resting on the tang that is either not perpendicular to the axis of the needle or there being a divot in the tang which can upset proper needle operation. The issue with needles is that new needles may not seal as well as proper Weber needles did years ago. I wet pressure tested with gasoline over 200 needles for Richard Parr (PMO) due to poor sealing even when new. I discovered an 8% failure rate for these new needles, 8% is rather terrible. I have since changed to using Viton-tipped needles to help overcome the erratic performance of these new needles. Even after testing these 200+ needles I would receive them back in carb rebuild kits from Richard and would still have an occasional leaker. Very troubling when a customer has a flooding float bowl on a set of Webers I just serviced...very troubling. Point being that even though I wet tested and then test ran my finished Webers on my test engine and verified running fuel levels I would get an occasional flooding issue from my customer. These issues occurred randomly, one day good and another day not so good. Viton-tipped needle valves plus my standard blueprinting floats and removing the divots on the tangs seem to have made random flooding and erratic running issues go away.

Of course, based upon my current occupation, I tend to color all things as Weber problems and minimize alternate issues.

[edmayo]

Ed - Not being aware of the Weber idle circuit / idle jet debris sensitivities a year ago when I first encountered missing, I didn’t check WOT operation to redline because of driving conditions immediately before noticing the missing. As I speculated previously, I may have mis-diagnosed the problem when I changed plugs the 1st time. It’s a mystery why operation went back to normal afterwards, only to begin missing again about 100 miles and 11 months later. Timing was “checked and verified” by the Porsche shop before I took delivery, and not touched since.

What’s the explanation for the coming & going of the missing somehow coinciding with mysterious timing changes that would have to have gone from good to bad, back to good, and finally back to bad if there’s a correlation between them?

I can't tell you from a distance exactly what is going on, but what I can do is try to guide you on a logical diagnostic path. We need to determine whether this is fuel or electric, whether it is confined to one or two cylinders or is due to an overall condition. This will involve cylinder drop tests via various methods and then going on from there to finally resolve this. Generally the more difficult the discovery is, the simpler will be the final solution. We deal with this kind of problem every day.

[1969911T]

We are taking guesses based on our experiences, but the only real clue seems to be that putting in new hotter plugs stopped the problem for 100 miles(probably about 2 hours of running). The only thing you mess with is the spark plug wires when changing the plugs. So, I would put the old plugs back in and see what happens.
Steve