911 Surging Idle

[hifeverz]

RE: '74 911 2.7L engine

During idling, when warmed up, the engine RPM constantly changes from 950rpm to 1150rpm and back and forth. Timing and dwell angle(points) are per specs. Rotor/cap/wires appear to be OK. NO crossfiring arcing seen between spark plug wires when its dark. It seems to run a little rough in higher RPM. I'll change the plugs again to see if that could be it.

Any hints of what else to check on?? . . . and some test shortcuts??

Thanks,
HZ

[warment]

My 73 MFI has the same problem.

Bill

[Zeke]

Originally posted by warment
My 73 MFI has the same problem.

Bill

For a much different reason. You might as well not even read the rest of this thread.

I could give you some of the things to do and look for, but that's some. I started with the butterfly adjustment and then the air correction screws when adjusting (in the sequence check, measure and adjust) and that made a huge difference. The car above has only one butterfly and the idle speed is adjusted with only one screw. I like my MFI, but some guys have it easy.

[stlrj]

CIS can get a rich idle hunt just like a DME when the CO is set too rich.

Joe

[hifeverz]

'74 CIS 2.7L

Adjustment of the CO is that accomplished with the allen screw located between the fuel distributor and intake boot??? Or adjustment in a different location??

Please advise.

Thanks,
HZ

[scottb]

My 84 had similar problems. It was the mixture.

[Paulporsche]

That's it.

Turn the adjuster counterclockwise (as seen from above) in TINY amounts, maybe 1/32 or 1/16 or so of a turn until the fluctuation stops. Rev the engine after each adjustment. Do not leave the allen wrench in the screw while you rev the engine after each adjustment.

If you don't have a gas analyzer, this can be risky for a newcomer. I have been doing this to my car for 15 years, so I know what to expect.

Since you are already rich, a small adjustment will probably be fine. The car will tell you when you are right. I find if I adjust it to just before the point at which the idle begins to hunt I get my best results. The car will rev freely to redline without feeling choked, or without pinging . I also find I get a very dark brown, not black tailpipe.

I find I also have to richen the mixture each fall when the temps and gas blend changes.

If you are unsure, review the threads about the Gunson Gastester. JW recommends a 3.5% CO as a starting point, but has said that you still need to do some seat of the pants testing to get it right for each individual car.

Good luck.

Paul

[warment]

Zeke,

I have made several adjustments to my MFI in the past. I checked the timing with a timing light, and I have a Gastester. I have not used the Gastester in quite a while. I may test the mix again this weekend. Everything else seems to be working fine. I have no popping, have great power, etc.

Bill

[hifeverz]

I went ahead and adjusted (successfully) the CO using that allen wrench. I have a Sun CO gas analyzer though I was too lazy to dig it out of storage.

I adjusted the screw approx. counterclockwise (as suggested) by about 15 degrees and now it idles smoothly.

Thanks for the tip,

Hz