My Porsche is possessed

[sceaf3]

My Porsche is possessed. It is a 1975 911 Carrera, stock 2.7 with stock CIS. I have had the following problem for about 2 years with no luck at curing it. There are times when the problem will not appear for days, other times it shows up one block down the street. It seems to show up more often when it gets hot, but will also do it to a lesser degree when only warm. The symptom is that it starts to jerk, lightly at first, sometimes so lightly that only I can feel
it. But I recognize it right away and know what is ultimately going to happen.
It may continue only lightly for the rest of the day. Eventually the jerking back and forth becomes violent then it either begins to backfire and then dies, or just acts
like it has run out of gas. When I say jerking, I mean it feels like the engine just quits but only momentarily. The jerking has no rhythmic cadence as it would with most ignition faults. At times it will crank right away after the engine finally dies; sometimes it takes a while to crank. One mechanic who started his career working on 1970s 911s thinks that it may be vapor locking. I don't see how a system that is under constant pressure can vapor lock when the engine is running. Further it can be present any any time. It gets worse as it gets hotter but not regularly enough to decide it is vapor lock. The car seems to run a little hot, but not excessively. It has a Mocal front oil cooler replacing the trombone. It gets up to 230-35 pretty quickly in 90+ degree Florida weather in town with the AC on. It cools of pretty quickly after getting on an open road and turning off the AC. Runs pretty consistently at 210-20 even on mild 82-85 degreesdays. All suspected electrical problems have been eliminated. Everything on the CIS system has been replaced or rebuilt from the fuel pump to the accumulator, to the warm up regulator, fuel injectors, gas tank replaced due to sediment build up, etc.
The mixture has been set and re-set. The only major part of the CIS system that has not be replaced or rebuilt is the fuel distributor. Because we found a good bit of sediment in the gas tank and replaced it I think some of the sediment could still be in the fuel distributor. The system was purged, but it seems to me it would not take much to make a fuel distributor malfunction. Question: Has vapor lock ever been a problem on this model? Do you think the symptoms could be caused by a malfunctioning fuel distributor? Do you have any other thoughts or suggestions for me



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[Steve@Rennsport]

Vapor lock has been a chronic problem on all these cars until Porsche finally relocated the fuel pump up front, like the 911SC's.

Don't overlook this critical detail.

If you think thats the source of your problem, its not difficult to relocate the pump under the front crossmember.

[Peter Zimmermann]

Sorry to ask a basic question, but have you replaced the ignition points, and set the dwell & timing? Also, I've seen this problem caused by too much grease on the distributor cam (which gets "thrown" onto the contact points), a careful cleaning of the points using a Q-Tip & lacquer thinner will take care of that (if the point dwell is in spec)! A thin smear of grease is all that's required. No grease is very bad, too much grease is almost as bad!

[Trackrash]

I have had similar problems. Turned out it was the rev limiting rotor in the distributor one time.

Another time it did the same thing, turned out there was some resistance in one of the electrical connectors that powered the ignition.

Do you have one of those electronic points replacements (Pertronix Ignitor) or points?

[Paulporsche]

Just to add to what Peter Z said, check your wires, plugs and cap, aside from first looking at the points and timing.

You say the WUR was rebuilt or replaced. Did someone actually verify the fuel and control pressures?

Then move on to the fuel issues.