Surging between warm up and really warmed up.

[DJM]

I have a 71 chassis with 82sc engine and its running badly.

During warm up, no problems. After 20 mins of driving, it's also running ok but with a small amount of surging about 200rpm.
The real issue is between warm up and about 20 mins. It surges so dramatically that it cuts out. Pretty dangerous as I try and keep the revs up as i slow down at lights etc. Also at this time, if it does stall, it is something difficult to start, having to wait 5 mins.

The issue has been there since I took the engine out and put it back. The story is:
It was running fine until about a year ago when the gearbox needed a rebuild. While the gearbox was being rebuilt I removed and reground the cams to a 964 profile. I didn't strip the engine back any further than necessary to remove the cams. Cam timing set to 1.3. I removed the CIS system with intake manifolds as 1 unit and replaced as 1 unit. I didn't touch the fuel mixture. Idle and ignition set ok at 950 when well warmed up.

I am presuming I've done something but I don't know what. I've looked for a vacuum leak but can't find one. When I remove the oil filler cap, the revs drop. I've sprayed carbie cleaner around and only get a change to revs when it goes in the air intake.

What's my next step to root cause the problem? Is it time to play with fuel mixture or should I check something else first?

[timmy2]

Finding CIS vacuum leaks - the vacuum cleaner pressure test

[theiceman]

you dont have a vacuum leak. you are surging because you are way too rich. i bet your plugs are black.
get an exhaust analyser and lean it out.

the other problem could be in the o2 sensor circuit.

[DJM]

Thank you for the advice. The fuel mixture was definitely out and when fixed stopped all the surging.
However, I'm still get this big swing of the revs down past idle and sometimes cutting out. Sometimes it swings past idle and then lifts back to idle and sits there comfortably. Other times, when I put the clutch in when the revs are greater than 2500, it'll swing last idle and cut out.
What do you recommend I look at next?

[boyt911sc]

DJM,

Is your '81 SC engine a USA or Row model? What's the number (ID) on your WUR? Check your fuel pressures versus time (0 to 10 mins.) from a cold start to WCP and record them including engine RPM. Inspect if the FV (frequency valve) is vibrating while the FP is running. Do you have access to a dwell meter? Refrain from tinkering the mixture setting and do it later. Take a good look at your spark plugs. Then you'll have an idea which direction to go next.

Tinkering the mixture mixture setting without knowing the fuel pressures and integrity of your vacuum system would only make a bad situation worse. So be patient and systematic in your problem solving process. Keep us posted.

Tony