Can't figure this one out

I just can't get my 78SC to stop dying when I come to a stop. I have the stock CIS (or "C"an't "I"mprove "S"peed much system). A week ago I adjusted the valves, replaced the plugs and changed oil. Immediately thereafter, the engine started right up and runs great. However, after it warms up the engine dies after I come to a complete stop. But it doesn't just plain die. After I come to a stop the engine will dip down to about 200rpms, jump right up to about 1300rpms, drop right back down to 300 rpms, then back up to 1300 rpms and immediately dies. However, sometimes it just drops down to very low rmps, evens out to 900rpms, and continues to run at idle (900rmps). The engine idles fine after I start it back up (again 900rpms). I've checked all the vacuum connections I could have knocked loose when adjusting the valves, and their OK. I've even attempted to turn up the idle, but again, to no avail. I'm baffled. I know there can be idle problems when the engine is cold, but I have not been able to find a thread that discusses this idling problem when the engine is warm. Any ideas?

Paul
78SC Targa

I suspect that you need to set the sensor plate height. I think you are a little lean. It runs fine when cold because the warm up regulator is enrichening things. But once the WUR is done with it's job, your base setting is too lean and that effects idle stability. Either that or it's that "COSMIC MYSTERY FORCE" Warren told us all about in that long since bygone thread about the stuck CV axle. You'll have to ask him about that one.

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Tyson Schmidt
72 911 cabriolet (964 bodywork)

Sorry DP.

[This message has been edited by BRAINIAC (edited 11-30-2000).]

You said that you "tried to turn up the idle but to no avail". Does that mean that you can't increase the idle speed above 900 RPM when warm, or that a higher idle speed doesn't help?

Have you adjusted the mixture while it's warm?

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Bill Krause
'79 911SC Euro

Sounds like "idle hunt" again. I have it too and it comes and goes on my '82SC and one of these days I've got to follow all the past forum threads and trace it down (it's gotta be somewhere in the CIS/OXS adjustments/circuitry.) When it "comes" I have to up the idle to get back to a steady 900rpm and when it "goes" the idle starts running at about 1800 rpm so I put the idle back down (thank god for the easy-reach, fast-action, finger-adjustable idle screw as it can strike any time and I can usually jump out at a red-light and adjust before it changes back to green ). Days or weeks later it returns again. Might be related to the weather (or humidity? or maybe too much pollen! --- a Porsche with allergies? ) but I haven't found enough of a pattern to pin it.

[This message has been edited by tog (edited 12-01-2000).]

78SC's don't have ox sensor or the lambda circuit. I think he just needs to get his mixture set when the engine is warm.

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Bill Krause
'79 911SC Euro

Mine did that...ended up being a leak in the intake boot.

Well, that's exactly right. There are quite a few opportunities in CIS intake systems for leaks, and this is a likely suspect with your symptom. If vacuum leak is not your problem then I'd look at the air flow sensor plate. It probably has a bunch of grunge on it anyway.

There is a spec for that, by the way, if Porsches are as good as VWs. The plate must sit in exactly the right position relative to the lip of the air cone, and you can just measure it if it's suspect.

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'83 SC



[This message has been edited by Superman (edited 12-01-2000).]