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Discussion Starter #1
I went through major engine rebuilding. I almost forgot how it is to drive my old red crocodile (we call this way classic 900 here in Poland :) ). Ebgine runs smoother and much more quieter but...still hunting while idling. And the clue - while I move the plug/cable going to the ignition distributor rpms also going down and up. Sometimes the car simply stalls. I took a look at Bentleys but still have no idea how everything works and how to check it. Could the faulty distributor cause erratic idling? Is this for sure distributor not the cable?

And one more thing. How long should I drive it very carefully? I changed my oil and filter after almost 1000 km, when the engine didn't see larger number than 2500 rpms. Now I'm not sure if I can take it more higher or not.
 

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The distributer on an SPG is very simple and only varies the timing with boost pressure so it is unlikely to be the problem. Have you measured the timing at idle, and if so does it vary much? Standard car should be running 16°. A small amount of jitter on this value is usual.

Its possible that is the AIC having problems (if fitted). Another possible is the mixture being massively out.
 

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Discussion Starter #3
Correct me if I'm wrong. There's a distributor. There's also a 3-pins plug. I am sure that moving it by hand that lead/cable causes stalling and variable idling. Is it for sure it's working only under boost?
 

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Saab Mad
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On a C900 there is no connection between the APC and ignition systems. The APC systems will lower boost pressures in the event of knock. Ignition timing remains unaffected.

Later DI/APC systems on 9000s will first retard the ignition timing when knock is detected, and then lower boost if necessary.

(I wrote the above in response to serre's post before he edited it)

Would the distributor's electrical connector be for the Hall effect sensor? If so, then a loose connection there would cause engine speed fluctuation...? The plastic connector on the distributor IIRC is known to be fragile.

Hunting though is normally associated with a vacuum leak, so perhaps there are two problems at work here.

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I agree with Matthew that the Hall sensor may be defective. I have known a Turbo 900 where the idle "hunted" up and down and a new sensor solved the problem.

Bartek, I suggest trying another Hall sensor. They are expensive, so try one from another car (if possible).
 

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Discussion Starter #7
Thanks guys! It's the answer I've wanted :). Is replacing distributor connected with some timing matters?
 

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Saab Mad
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If you replace the distributor then you'll need to re-time the ignition.

Find a fixed mark or point on both distributors, and mark off that point against the cylinder head. Then then you fit the new distributor, line up the fixed point or mark with the mark on the cylinder head. That will hopefully get the timing near enough to start the car.

You then must re-time the engine accurately using a decent timing lamp. Don't forget to pressurise the distributor's vacuum capsule to ensure that timing will be correctly adjusted when on boost.

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