CIS Erratic Idle
 

I have a 1983 engine with a 1978 CIS on top of it. When starting it up, it runs, revs up, then tries to die. It does this rev & drop cycle repeatedly regardless of the idle screw setting. Eventually this cycle will end with a backfire and the engine dying, unless I flex the fast idle lever in the cockpit back enough to keep it revved.

I've tried gently pushing up on the airflow sensor plate, but added fuel does nothing to help the situation. Fuel is spraying from all 6 injectors equally (visual inspection).

My gut tells me this is an air leak. Please respond with any other experience (or gut) based opinions on what this could be.

Thanks!

Do you have access to a cis fuel pressure tester? If you suspect an air leak, try spraying carb cleaner in and around the suspected area. Were there any symptoms prior to this or was there any work done to the motor? I'm not sure lifting on the air sensor plate will do anything to help troubleshoot your problem.

Do you havea pop-off valve? If not, did you experience a backfire recently?

You might be too rich. Instead of lifting the sensor plate, try holding it down a little. That surging idle is a symptom of a overly rich mixture.

use the long 3mm allen wrench to manually turn the mixture leaner, c/clockwise, until the surging just stops. it may require up to 1/2 turn in some instances.

I think you're right about the air leak and I know there's been a ton of posts on this issue. My '81 was doing the same thing as your car and I did all the usual tests that I felt comfortable/adept trying on it. I finally gave up 3 weeks ago and took it to my 911 guru in Tampa.
He's been working on it "occasionally" since I told him I wasn't in a hurry (probably a mistake), and he hasn't been able to find the leak either. He did make the start cycle better by richening the mixture, but now it won't idle under 1800. I'll let you know when we find the culprit.

I had this problem with my 1974 CIS 2.7. After reading many posts on this issue, I disconnected the vaccum hose to the decel valve and I have had a consistent 950rpm idle, no surges etc. Many of the recommendations are to plug the vaccum hose, when I did I still had the same results, I left the hose unplugged and it drives better than it ever has. Not sure if this is the healthiest thing but it's been 6 months and no problems.

Adjust the idle screw very slightly then rev it a bit...remove the 3MM wrench first, don't leave it in there!! A 3MM allen wrench from Sears will work fine...no need to mail order it from all the possible sources then wait for it to arrive, unless it's Pelican of course.

shakenbake,

By disconnecting the vacuum to the decel valve and not plugging the line, you have taken vacuum away from the WUR. You may have fixed your rich condition by making WUR create a leaner condition. I'm nut sure this will hurt much, but you are sucking unfiltered air into the motor, adding air to lean the A/F out a bit. You are also operating your WUR in a none spec condition.

I would connect the decel valve and lean out with the mixture screw to see if that doesn't work for you...just a thought.

Good luck!