CIS Idle adjustment

[Johnb911]

I just purchased my first 911.
79 EuroSC.

Been dreaming of it for 24 years now. Since I was 12.

I purchased the car from a fellow in kelowna BC, which is a higher altitude than Vancouver where I live.
The car idled well there.

I brought it home and it now idles at about 1300 rpm.
So I made the fixable mistake of jumping in and adjusting the idle mixture without realizing what it really was doing.

Since I adjusted the idle mixture I failed the idle test at AIRCARE.
I have read the article by Souk about idle speed and fuel mixture and have a much better understanding of what is going on.

I will post my results so that someone can maybe tell me how far out I am with the idle mixture adjusted so that it idles at about 900rpm but fails air care and If you are willing give me a bit of a step by step on how to correct.

At Speed test
HC 82.00 PPM
CO 1.19%
NOX 835PPM

Idle Test
HC 256PPM
CO 5.43% FAIL Must be 4.44 or lower.

Is the whole car too rich or just at idle?
If I adjust both the mixture and the air by pass what does this do to the overall car.
Am i better to turn the air bypass up again and go through with higher idle and then take to mechanic?

Newbie Help, Please.

I would like to learn these things and be self sufficient if possible.

John

[MotoSook]

The at speed mixture is really determined by the design of the system (meter venturi, fuel system pressure and control pressure). You adjust at idle, and everything should fall into place unless your control or system pressures are out of spec (some out of spec is OK).

What is the HC limit to pass?

Adjusting the bypass screw will help control idle speed. Adjust the idle mixture using a gas analyzer (4-gas is best, but CO monitor is good too).

You can try the "field idle adjust" method, but it is not going to guarantee you pass. Set it lean...turn the screw CCW until you start to hunt, then go CW a 1/8 or 1/4 turn to stop the hunting. After that is done, decide if your idle speed is acceptable. If not, change it to what you want (try 1000 RPM) and then re-adjust mixture.

With a 4-gas analyzer I had to spend almost an hour to get mind to acceptable levels. That include setting it (mixture screw, bypass screw, and ignition) and driving it to settle it. You may not need to do that, but my CIS is a bit of a bastard, so it took a little more finesse.

Edit: WELCOME TO THE BOARD!

[Eric-325I]

Grats John!!! Bought my '79 SC silver over black a few weeks ago and I'm still making special trips to the garage to look at her.


Welcome to the best Porsche board on the web... Great people here!

Eric

[bkcbuilder]

Congrats fellow 79' Euro owner...mines emerald green.

I had similiar problems and was completely baffled and more frustrated than Ray Charles with a "where's Waldo book" but then came to grips with the fact that I could not tune THIS car by ear, and gave my ego a weekend vacation and bought a fuel pressure gauge set and a CO meter.....and the CIS book.
Tracked down an air leak(hunting idle) then set the CO and idle in an hour or so....been in heaven ever since. I know many guys whine about the CIS(and at first I did too) but when set correctly it is great, and pretty much stays that way. I haven't had the CO tester out in over a year, pressure guages either.

Oh yea....obligatory Newbie Probie car pictures are required...post them immediatly. Brian

[azasadny]

bkcbuilder,

Where was your air leak?

[bkcbuilder]

One of the rubber boots from the air box to intake runner. Could not see it, but when I hit each rubber with some WD-40, it idled up fast when I got to that rubber...sure enough when squeezed the crack opened up.

[kach22i]

When I tried the starting fluid test the engine did not increase in speed at anytime.

Does this mean that I don't have any leaks?

Or does this mean I have very tiny leaks?

[azasadny]

Thanks for the info!