Easy way to diagnose a erractic running 911?
 

I've got a 74 911 with an 81 3.0 CIS engine. The car cranks fine cold warm and hot but has a miss at idle and during acceleration almost like it's running on 5 cylinders. The car is a daily driver and hasn't been sitting for long periods of time. Is there an easy way to troubleshoot this to determine whether it's a ignition, or fuel system or other problem before trying to replace parts without knowing. Thanks all.

start with the plug wires first, make sure they are all connect and in the correct firing order

Yes the plug wires are in correct firing order. As a matter of fact both plug wires and spark plugs are like new.

Once warm, at idle, spray the individual exhaust pipes by the heads with windex or the like and see which one is cold. Once you determine the culprit, then you can look to spark, fuel or compression.

Bruce

If your car is in the air that is a slick idea.

I pull the leads from the coil one by one to see which one causes the engine to not run worse. Pull the wire while the engine is off. :-o

Run the engine in a dark garage. Look for arcing plug wires. It's quite a show.....

I had a rough idle/rough acceleration below 2000rpm problem on the 78sc I just purchased. I adjusted the mixture a hair richer and that solved both problems. I did not have an exhaust tester so I just slowly tuned it until the idle smoothed out. Then went for a test drive and all is well.

Does you 81 SC engine have a working lambda ECU and FV ?