1980 911SC CSI problem

[Bgrenfell]

I have a 1980 911SC. I have had the Bosch ignition distributor rebuild and the WUR rebuilt. The car runs great on cold start but once warmed up at about 3k rpm the engine wants to sputter and lurch. Once I get it past 3k it runs better but still has an occasional hesitation. Help please!

[Funracer]

Has this problem surfaced after your WUR was rebuilt or was it there beforehand.

[PeteKz]

List how many miles, other recent work, and any mods (including ignition). Also, add your location to your profile.

Offhand, if it runs good cold and starts stumbling when warm, then it's probably going lean in that rev and load range after it warms up. Does the problem occur at 3000 whether you are hard on the pedal or light on the pedal?

[Schulisco]

Originally Posted by Bgrenfell

I have a 1980 911SC. I have had the Bosch ignition distributor rebuild and the WUR rebuilt. The car runs great on cold start but once warmed up at about 3k rpm the engine wants to sputter and lurch. Once I get it past 3k it runs better but still has an occasional hesitation. Help please!

My opinion: Wrong mixture! I suspect there are one or more massive vacuum leaks! If the car starts good being cold and there's no sweeping idle then the mixture is okay when being cold, but it get's too lean / too rich when warming up. To verify this you need a AFR gauge. Also check with a smoke generator for vacuum leaks. Brake cleaner is not precise enough to find every tiny leak which will make a difference on a CIS engine! Then check control pressure from WUR and ignition timing, and check if the vacuum hoses at the distributor are connected correctly and if the right outoputs used at the throttle body for the distributor! This is an often, but unidentified error.

Thomas

[PeteKz]

Thomas, you spoke of using "brake cleaner" for testing for vacuum leaks. I hope that was a mistake, and you know NEVER to use brake cleaner on a running engine. Carb cleaner, yes, brake cleaner no.

Why? Because most brake cleaners (at least in the USA) are chlorinated solvents, not straight hydrocarbons. Under high temperature, they break down and one of those breakdown products is the WWI nerve agent, phosgene gas. About 10 years ago, a prominent technical writer for a motorcycle magazine had cleaned some metal parts with brake cleaner prior to welding them, and then proceeded to weld them. Some of that chlorinated brake cleaner was still in the seam, and the heat of welding created phosgene. He lived to write about it, but still had long term problems from that small exposure. BTW, cholrinated Freons will also generate phosgene under the right circumstances.

Here's the link:

https://www.brewracingframes.com/safety-alert-brake-cleaner--phosgene-gas.html

[PeteKz]

And I 100% agree with Thomas that vacuum leaks cause lots of running problems with CIS, especially at 40+ years of age.

[Schulisco]

Pete, you're right, thanks for the hint again. As I wrote in this posting I suppose that meanwhile almost all available brake cleaners are free from chloride, acetone and aromates...also not to mention that it must be used out of enclosed rooms only! Nvertheless - start spray is the better alternative to detect starting issues...no question.

https://forums.pelicanparts.com/porsche-911-technical-forum/1135135-cis-cold-start-trouble-ready-new-ideas-2.html#post11963958