Won't run under 4000 RPM

[Thack]

My '79 911SC will absolutely not run under 4000 RPM. It will start, sputter and backfire under 4K. It does run really well over 4K RPM which takes full throttle. So well that I didn't know my car could run that fast. I also have to prime the air intake with some fuel to get it started. Where should I start and what are the possible causes.

Mike

[unclebilly]

Clean your CIS system with carb cleaner. It seems like the flap is sticking so unless you have it matted, it won't work at part throttle.

[john70t]

It does sound like a fuel problem.
The only other thing I could think of would be the timing advance being stuck in the full position, but that would give a high idle and knocking and not a spitting misfire.

Lucky you, time to learn about CIS distributors, the injectors seals probably are ready for refreshing anyways.
Take injectors out and jump relay/put in glass jar/check spray pattern and plunger smoothness etc...etc... it's in the archives.
The CIS needs a special pressure test kit for checking control and system pressure. Harbor freight is one place for a cheap kit.

[Grady Clay]

Mike,

Could be a broken air box from an intake backfire. While it is running, grab the air cleaner and lift, move it around, etc. If the air box is broken, the running will change.

Hope not.

Best,
Grady

[svandamme]

had the same thing on my 924

turned out to be electrical ,
so the idling injector didn't work

[Thack]

Thanks for all the tips. I was going to start with the fuel filter or tank trap. I'll let you guys know what I find out.

Mike

[David]

Buy or borrow some CIS fuel pressure gauges and check your pressures. If you're going to do your own work, you really need a set or access to some.

[Thack]

I think unclebilly was right. I moved the sensor plate up and down and it started. I thought it might be the frequency valve but it started right up and revved just fine from idle to 4K. I'll clean my plate thoroughly with carb cleaner and inspect everything when I can. I sprayed some cleaner after it started just to ensure it wouldn't stick again. The funny thing is it didn't seem to be stuck?

Mike