engine missing at idle and under load
 

Hello clan of the flat six-- I've learned a lot browsing this great website, so maybe someone can steer me in the right direction on my current problem. The car is a '75 911S with a pretty solid engine, but I recently did a partial drop to fix some oil leaks and switch to old style heat exchangers. Since getting it back on the road, it's been misfiring quite a bit. No backfires, but it will really shake and shudder at partial load. Also misses at idle. I've double checked the points and pulled all the sparkplugs. They look very clean to the point that I'm wondering if the mixture is too lean. During the partial drop, I replaced blown out gasket between the air box and the cone/distributor assembly. Yes , I now have a pop off valve. More info: valves adjusted 3,000 miles back; car sat for 5-6 months while I was working on it (old gas a problem?); not sure if the CD unit is working (I don't hear the high pitched noise it's supposed to make) is it an all or nothing type unit when it goes bad?);I've also been suspicious of my fuel pump. Any comments will be greatly appreciated, as I'm 65 miles from the nearest mechanic.

i had similar problem last january...it turned out to be a wire on the points that was shorting out because it was rubbing on the cam of the distributor...i originally thought it was the point gap....i changed to a pertronix electronic ignition and have not had any problems. i have an msd 6al in lieu of the cd so i cant help with that. remove a plug wire and plug it into a new plug and hold it against the fan housing do you get a good consistent spark?
check for vacuum leaks also by spraying carb cleaner around the filter box etc.
if you were having a fuel pump problem i think it would be more recognizable at higher rpm's.
a consistent miss is normally a spark problem in my experience.
why did the gasket blow out?
the mixture could also be the problem i guess, but i would put the car in time, make sure you have good spark, check for vacuum problems and then adjust the mixture.
gary

Thanks, Gary---I'll be checking out the items that you suggested this week.

Did you just put in the popoff valve? Could it be seating incorrectly or lack a proper seal at the airbox?

John

I had a slight miss at idle and under light load from a take off. Not exactly as you described, but worth mentioning. I tracked my problem down to the popoff valve I had just installed. I didn't pack enough epoxy around the hard to reach back side of the valve.

Sounds like you either swapped an ignition cable onto the wrong plug or your cables are shot. I would check my firing order on the distributor cap and make sure the wires go to the right plug, then turn out the lights in the garage with the engine at idle and watch the light show or listen for the snap the spark makes when it jumps to ground.

It has to be ignition related.

Cheers,

Joe