idle problem
 

After tinkering with my 1980 911sc for a year it runs really well except for a small idle problem.

At cold start the engine will idle down from 1800 rpms to about 700 and then back up to 1000rpms as the engine warms. Apparently the AAR is working well. It runs smooth and pulls strong throughout the power band.

I searched the archive and found a couple of threads with a similar problem but no resolution was posted.

On several ocassions I set the CO with a gunson gas tester. last tested at 1.4% a little richer than spec. I run the cat with the oxygen sensor connected.

The timing is set at 5 degrees BTDC.

I am lost as to what to trouble shoot next. Would the warmup regulator cause this variation in idle. Thanks!

Assuming you set the CO with the car fully warmed up, then, yes, it's possible the WUR is off, making your car too rich when cold. If you check or have checked, your control pressures at both cold and warm, you may find that your cold control pressure is too low (rich) while your warm is right.

Paul

Paul

The CO was set with the car fully warmed up. i have also noticed a slight backfire when I let up on the gas during the warm up period. Would that not indicate a mixture too lean? but your explanation makes sense. At any rate, it looks like I will need to add another tool to my collection. a fuel pressure gauge. thanks for the input.

roger

I had a similar problem on my 84. It was the mixture.