CDI starting to fail or something else?
 

Car is a 1976 911S with rebuilt 2.7.

Engine has stock injection and ignition. The engine was rebuilt 3300 miles ago and runs great. All new plugs, wires, rotor, cap, etc.

Was out for my normal weekend drive this morning and all was right with the world. Had been driving around 30 minutes and was on a flat level road turning around 2900-3000 rpm when I thought I detected a misfire. Engine was running fine so thought it was my imagination. A couple seconds later I had a real misfire where the engine rpm dropped for a few tenths of a second. Then the engine started running normally again. A few seconds later engine misfired again (but not as bad) and the engine resumed running as normal.

The car ran fine the rest of the way home (around 10 miles) and exhibited no other symptom.

I checked the cap and rotor and the plug wires. All look good. Is this failure mode typical for CDI? I believe mine is original.

Not sure if that's an indication of a failing CDI or not (mine hasn't failed yet).

I had some similar symptoms that I later attributed to worn distributor bushings. A distributor rebuild took care of that.

You might also check points (if you still have them) and the vacuum module.

When my CDI failed didn't gave symptoms... Simply while running engine backfired and then stopped... No way to start it again until a new unit.
As told by fanaudical, check your distributor!

Bill, check your dwell as the points may be collapsing.
Bruce

Sounds more like a points gap issue to me...

Spare
 

Before my CDI went on my 81, had two similar misfires to what you mention. Finally after running a few weeks later on a hot day with lots of idling and heat, it started cutting out then finally died with no restart. Only time I had to get towed in 13 years.

Don't mean to scare you unnecessarily but it may be getting that time. If on original you should consider carrying a spare anyway. When that goes, you aren't going anywhere without spark. After 100 bucks plus for a tow, I now carry a spare for long trips.

Thanks, guys.

I'll check the distributor and see about getting a spare CDI and see what happens.

CD or coil can do it.
3300 miles, the points should not be an issue unless the gap was originally set way too small.
the gap is not critical, make sure it opens and closes. I always eyeballed mine and they were fine.
the points just provide a ground, no voltage so no wear on the contacts.

grounds from engine to body and CD to body. check them.

Had the exact same symptoms and it was the CDI.

I had similar symptoms and it was ignition crossfire. I diagnosed the problem by carefully cleaning the distributor cap and seeing the problem disappear. I then replaced the cleaned cap with a new one. A light coating of dust, a thin oil film or other debris is all it takes for the spark to take an unintended course across the cap.