1981 SC Will Start But Wont Idle - Setting Mixture
 

A couple weeks ago I won a 81 SC on a popular online auction site and was super excited about getting back into older 911s after being away for over a decade. It showed up quite different that what was described, however that isn't the purpose of this post.

First project is to get the WUR, mixture, and idle tuned because it is obvious that the car was setup without pressure gauges. Originally the cold pressure was .93 bar, don't remember the temp but it is California, but that isn't even on the WUR chart. I believe due to the pressure being so low that they had leaned out the mixture and the idle was almost all the way screwed in to just get the car to run.

Currently adjustments since my car was delivered to me:
- Cold pressure at 1.9 bar @ 20.7c, which is the middle upper range
- Idle Screw counter clockwise 2 1/4 turns (@ 1/4 turn at a time), which is a little high, 1400 - 1600 rpms
- Mixture Screw clockwise 3/4 turn ( @ 1/4 turn at a time)

The problem is that I can get the car to start but it won't idle. If I stay on the gas it will run however when it idles it almost immediately studders and dies. I set the idle high to make sure that it wasn't a low idle situation knowing I would tune it down later.

This is the interesting part. After I set cold pressure and didn't touch idle or mixture, I started the car with my fuel pump jumper in place. It was a total accident, got too excited, and when I realized it immediately shut the car down. However when that jumper was in place the car started and ran well, albeit a little rich by testing via lifting the place and hearing a stumble. I know the car isn't meant to run in this condition but I wanted to add this as an accidental data point.

I believe I still have a mixture problem, I assume too lean, so what is the best method to attempt to get the mixture in the ball park with a car that won't idle? I know the mixture screw is very sensitive but do I need to keep going counter clockwise? Could this be indicative of a bad fuel pump relay or other problem? Any tips or advise would be greatly appreciated.

You have to unplug the loxygen sensor to make changes in the system.

Leaving it plugged, the system will not permit adjustments.

Bruce

This is stated in a way that is confusing.

You can adjust your 3mm screw and it will impact the system. However, with the O2 system functioning, it will compensate for your adjustments to the point it is able via the frequency valve adjustment to control pressure.

There is a good thread on how to dial in your CO without an analyzer. I would kill the O2 connection for now as every variable you can remove will make things easier. You have a can of worms now.

I agree that removing variables is the name of the game. I will disconnect the O2 sensor and report back.

(The problem is that I can get the car to start but it won't idle.)

Turn the mixture screw clockwise until she keeps running.


Cheers,

Joe

Does your car have the cube relay for the sensor system under the passenger seat? My 1980 SC does. The idle was terrible. I went through the recommendations of checking fuel pressures, WUR, etc. Before I touched the mixture screw, I replaced that $10.00 relay and the car started right up and ran perfectly.

Sounds familiar.

These engines are very sensitive to false air, incorrect fuel pressures and weak spark.

My car ran like crap when I first got it. A tune-up with new plug wires, plugs, cap, rotor, etc improved the spark quality. It did not fix all the problems, but it was a piece in the puzzle.

At one point I took it to a shop to do a smoke test to look for vac leaks. Without going into details on the whole story, I am not certain they actually did it. Here's why. They told me absolutely no issues found. A few months ago I dropped the engine and replaced the intake runner boots and gaskets, fuel injectors sleeves and o-rings and all the vac hoses. When I started the car for the first time after doing all the work the idle AFR was 10! The point is that there are lots of opportunities for false air and it has a huge impact on AFR. After resetting the AFR to 13.2-13.5 (CO 2-3%), adjusting the idle, adjusting the timing and plugging the idle retard vac line, it is like a new car. Now it pulls like a freight train!

+1 A possibility but not a slam dunk.

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He already touched the mixture screw but gave up too soon and if it works, it won't cost him cent. I just hate throwing money at a problem before doing the simple things first.

I say try your recommendation in post #5.

Can't hurt a thing and may be a remedy to get it in the ball park.

If he can get it to idle, just pull the cube relay and see if it makes a difference. If it does make a difference, that variable has been put to bed.

I thought we always started with the dome light over the passengers side door as that's on the same circuit as the Lamda box relay.

So the circuit is live and therefore that guarantees the relay is working, and the twelve pin connector is connecting to the freq valve and the O2 sensor is functional?

Is that what you are saying?

A dwell meter is the best way to adjust your mix.

It is roughly equivalent to a real CO sniffer and it's cheap. I have ONLY used a dwell meter in 18 years of ownership. Like Bruce says in post #2, Lambda system will make it right even if off a bit.

OP is not there yet.

No, what I am saying is that it is my understanding that the passenger' side dome light is on the same power circuit as the Lamda box relay (under the passenger' side seat) and that is the "usual" first place to start to see if there may be power to the relay. Never said much less "guaranteed" anything about the rest of the Lamda system functioning from the relay socket onwards.

OK. First check would be see if the circuit is working.

I was wr...

What the hell. I can't finish that sentence. Must be the Pelican server.

some things here are confusing.
you say it is too lean but you keep adjusting the mixture CCW, that leans it out even more. CW riches the mixture.

the PO had the idle screw ALL the way in to make it run? that could be a sign of an air leak.

you started it with a jumper in the FP relay and it ran better? it does not hurt to run it like that. you can also remove the connector on the back of the AFM to make the FP run with the key on. does not hurt to leave it like that when working on it.

here is what i would do:
make sure fuse 18 is good. thats power to the relay for the freq valve. make sure the FV is vibrating.
check warm control pressure and system pressure.
have someone start it and while pushing UP on the sensor plate behind the air filter.
if pushing it up makes it run better, it is too lean. adjust mixture CW.
this will at least get it running.

you need to check for air leaks.
does it have a popoff valve? is it seated? is the air box blown if it does not have one.
check intake bolts. injector Orings. vacuum hoses. the hoses tot he oil tank are under vacuum, make sure they don't leak including the oil cap.

check ignition:
cap, rotor plugs and wires.
set timing
make sure the advance works

once all this is done go back and set mixture and idle.

T77911S, you caught a huge error on my part... I have turned the mixture screw 3/4 turn clockwise. Sorry for the confusion.

So I unplugged the O2 relay from under the seat and tried to start the car. It would fire then immediately die. I tried that a couple of times then tried plugging the O2 sensor in and had the same behavior.

I then put the jumper back in to run the fuel pump for 20 seconds without trying to start the car. Then I unplugged the O2 sensor and tried to start it. It fired right up and ran really good. I went about making small tweaks to the mixture and idle to bring it into spec, the idle was high and was a little rich. At a point the car died. Then I attempted to start the car and was right back at square one. Cranks and dies. No matter what I did it behaved the same, O2 plugged in or not. Given this new info any tips?

Post #5 references leaning out the mixture.

Slow down a little. You are doing many things without feedback closure. Example, you pulled the relay a couple of times then pulled the O2 sensor. OK. What happens when you plug the relay/O2 sensor back in? Same?

Bob, I appreciate the help. With the O2 sensor unplugged or plugged in I didn't notice a difference in its ability to hold idle. It still started then died immediately.