79SC RPM Search during warmup

[Jaymanbikes13]

I have a 79 911SC and I am chasing an issue on warmup. I have had the car for about 8 months and I am new to CIS so just looking for what might be the next logical step.

The car starts without issue and idles at around 1100 RPM. It slowly starts to come down, after a minute or two the idle starts to surge. As the car warms the RPM's settle at approx 800 -900 RPM and the surge is gone.

I have done a smoke test and have only found one small leak which appears to be coming from the back of the airbox.

I checked fuel pressures and found.

WUR # 0-438-140-089

System Pressure 4.8B @ 31C
Cold Control Pressure 1.6B

10 minute test with the WUR Connected

1M 2.2B
2M 2.4B
3M 2.5B

The pressure does not increase after 4M

Residual Pressure
@0 = 2.9
@15M=1.6
@30M=1.35

Thanks for any advice.

[PeteKz]

The pressures look okay. Check your timing. Too much advance often causes unstable idle and bucking in gear at low RPM.

How often do you drive it hard? Also try pouring a bottle of Chevron Techron Concentrate into the tank, then go drive the piss out of it.

Report back what you find.

[Jaymanbikes13]

Originally Posted by PeteKz

The pressures look okay. Check your timing. Too much advance often causes unstable idle and bucking in gear at low RPM.

How often do you drive it hard? Also try pouring a bottle of Chevron Techron Concentrate into the tank, then go drive the piss out of it.

Report back what you find.

I drive it alot, will check the timing and let you know. I did check my Vacuum advance and noticed that its not working (sucked on vacuum tube) it appears that the diaphragm is no longer functioning.

Thanks

[Alan L]

It may be the diaphragm is working but the advance mechanism is frozen.
It may be the idle mix is slightly out.
Alan

[PeteKz]

Alan makes a good point. If the vacuum advance isn't working, or the distributor is stuck internally, then fix those first. Refresh my memory--does your dizzy have a vacuum can with TWO nipples (one for advance and one for retard at idle/closed throttle) or only one?

[Jaymanbikes13]

Originally Posted by PeteKz

Alan makes a good point. If the vacuum advance isn't working, or the distributor is stuck internally, then fix those first. Refresh my memory--does your dizzy have a vacuum can with TWO nipples (one for advance and one for retard at idle/closed throttle) or only one?

It has only one nipple on the vacuum advance.

[PeteKz]

That's the "good" dizzy. Make sure it's advancing correctly.

[Jaymanbikes13]

Originally Posted by PeteKz

The pressures look okay. Check your timing. Too much advance often causes unstable idle and bucking in gear at low RPM.

How often do you drive it hard? Also try pouring a bottle of Chevron Techron Concentrate into the tank, then go drive the piss out of it.

Report back what you find.

I checked the timing and it was about 5 degrees advanced. The vacuum advance diaphragm was completely shot, it wouldnt hold any vacuum. I removed the distributor and the vacuum advance, it very all really dirty inside, I cleaned it up and oiled it and installed a new vacuum advance unit, it works but guessing the distributor is gummed up inside because it doesnt move the plate when vacuum is applied. I reinstalled the distributor and now the car no longer hunts for idle at warm up as it was doing. I don't believe however that the distributor is working as it should. I set it at 10 degrees advance and it runs great. I guess I need to look into a replacement for the distributor, be it a Bosch or a 123 brand.

[ammonman]

I had to completely remove the pickup coil/advance plate and get it working again as the unit on my '78 was completely frozen. I was able to clamp the base in a vise with padded jaws and apply penetrating oil to the center hub and bearing. I was finally able to get the top half to start turning by hand but it was very stiff. After about 30 minutes of more penetrating oil and exercise it moved freely. I think a lack of previous owners adding a drop or two of oil to the hub bearing once a year allowed the grease inside to harden. Night and day difference in performance with a working vacuum advance.

[Jaymanbikes13]

Originally Posted by ammonman

I had to completely remove the pickup coil/advance plate and get it working again as the unit on my '78 was completely frozen. I was able to clamp the base in a vise with padded jaws and apply penetrating oil to the center hub and bearing. I was finally able to get the top half to start turning by hand but it was very stiff. After about 30 minutes of more penetrating oil and exercise it moved freely. I think a lack of previous owners adding a drop or two of oil to the hub bearing once a year allowed the grease inside to harden. Night and day difference in performance with a working vacuum advance.

I ran the car this morning and it was running pretty good at about 5 degrees advance, I dialed it up to 10 degrees and was running even better. Later in the day I went to start it and it didnt like timing at all, I had to dial it back to get it started. I then popped the cap and put my vacuum pump on the advance and noticed it was now moving. Guessing that my clean out job and lube finally broke it loose and thus why it was not happy with my timing.