CIS problems
 

Good morning America,
I'm calling from Belgium because I have problems with my 911 1973.5 with CIS injection (K-Jetronic). When releasing the throttle, the engine allways stops turning no matter cold or warm. Starting is easy with some throttle, but when you take your foot from the throttle-pedal, it stops turning. Any one any idea?
THX in advance.

Tough Question. There are some real pro's here on the CIS system, but it is not entirely clear you have an issue there.

Has the car run normally prior to this? Any recent changes that could affect this?

-Kelly

Air leaks are the most likely culprit. After that, I would look at centering the metering plate. Everything works back from there.
Center the plate, adjust mixture set idle.

On rare occasions, I will disagree with Henry. Intake air leaks frequently cause starting and running problems, but the metering plate does not come out of adjustment in normal use. If no one has messed with the metering plate or CO adjustment screw since this problem began, do not try to adjust those yet. Most likely, something else caused the problem.

Follow up questions: As kmcteer asked, was any work done recently that may have caused the problem? Did it happen suddenly or gradually? Have you checked the timing yet? Does the distributor RPM and vacuum advance work?

Check all those, and if you still have a problem, try to adjust the idle speed.

Then check for intake air leaks with a "smoke test."

Tell us what you find.

For your reference: https://cis911primer.com/home.html

Disagree all you want but a hard backfire (CIS engines are known to do that) can easily move and sometime even bend the metering plate. It was common enough for Porsche to offer a tool just for adjust the metering plate.
It's just a place to start that is easy to check, costs nothing and completely non-invasive.

Fair enough. if the OP already has the intake boot off the throttle today and air metering unit, it's easy to check.

Pieke, tell us a bit about the car. It is difficult to answer otherwise. All we know is that it starts well, but stops when you lift off the gas pedal.
Does the engine run properly with the pedal depressed? If so it may just need the idle screw adjusted. Is the hand throttle by the parking brake working? If so will it keep running with it lifted? The plastic piece connecting to the throttle linkage breaks over time 901 424 317 00.
How do the spark plugs look? Do all the cylinders have good compression? If it runs poorly, when was the last real tune up and valve adjustment? Spark plugs, wires, cap, rotor, points, been done recently? Is the distributor in good shape? Many have issues if the felt pad in the shaft was not oiled on occasion. The advance weights could be sticking. Play in the shaft can be a problem. Check with a timing light to see if ignition timing is in the ballpark.
If the basic tune up is good, then I would start looking into the CIS system, doing the checks mentioned by Pete and Henry.