Surging Idle

After my 78sc reaches operating temperature the idle surges slightly its only about 50 to 60 rpms but with a quiet muffler now on the car it is really annoying. I had the CO adjusted when it was tuned up and I did have a performance muffler on at the time would this make a difference. I was also thinking vacuum leak too. Anyone have any thoughts on the matter...Thanks, TargaEuro

set CO to3.5%, timing to 35 degrees at full advance, and idle speed at 1000RPM, and see if it still does it. you can set it at factory spec, or set it where it runs good.

[This message has been edited by john walker's workshop (edited 03-31-2001).]

Thanks Ill give it a try. Im not a Porsche mechanic by any means but shouldnt there be a vacuum line going to my dist adv mechanism. Its been plugged for some reason. It almost looks factory. This could be the root of some of my problem, maybe? Its going to a profesional mechanic in a week so Ill give him any leads that I get here to help speed up the diagnostic proceedure. Thanks everyone...TargaEuro

the distributor has a vacuum retard, so a lot of folks don't use it. just make sure you have 35 degrees total advance.

Thanks for the advise, I always thought that it was a vacuum advance. Ill give this info to my mechanic. Its to bad I live in New England you seem to know what your talking about when it comes to Porsches...TargaEuro

if it still works, put a hose on it and suck. if the idle goes down, it's a retard, and if it goes up it's an advance. these did come with a retard.

John -- I'm confused about the SC distributors. Which ones have vacuum advance, and which ones have retard? Where on the throttle body should the vacuum line attach for either type?

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Bill Krause
'79 911SC Euro

I'm confused too...

I have a '78 911SC and when I suck on the vacuum tube to the distributor the idle rpm goes up. My vacuum tube is connected to the top of the throttle body.

Tony

Ive done a little home work today and as far as I know 78sc's has a vacuum retard mechanism. You will only have one nipple for a hose coming out the front of the mechanism. If it were an retard/advance mechanism you will have a nipple out the front for vacuum retard and a nipple out the rear of the mechanism for vacuum advance. Retard/advance mechanisms are generally found on 80-83sc's. I think everyone should still check theirs for good measure anyway, this seams to be a very foggy issue at the moment. I have the Porsche factory workbooks in front of me but they dont seem to really Idendify this to well either. I know the 78 has a mechanical advance and like John says you should have 35 degrees of advance at 6000 rpm...Someone please correct me or add to this if Im incorrect in anyway...TargaEuro

As for the hose from the distributor on the 78sc's, I believe it should go to port vacuum, meaning below the throttle plate but on the throttle body...I think Ill do more research since I cant get to sleep anyway, Ill be back..TargaEuro

In volume VI of the workshop manual it says for 78 models, Ignition control: centrifugal advance and vacuum retard. This is found on page 0.1-7/1 chapter 9. Starting with 1980 the sc is equipped a double vacuum unit for retard and advance ignition control, thats from the very next page. That clears up that mystery. Now we need to find exactally were these hoses connect to....Back to the books

Doesn't quite clear up the mystery. Does it say what is installed on '79s? All of the diagrams I've been able to find show the vacuum line from the distributor going to the top side of the throttle body, indicating a vacuum advance system.

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Bill Krause
'79 911SC Euro

Where the distributor get the vacuum from matters but it doesnt change the action of the mechanism on your 79. You have a vacuum retard and a centrifugal advance, the same as my 78sc. Unless your distributor has two nipples for vacuum lines, one out the front of the mechanism and one out the back of the mechanism than you only have a vacuum retard unit. 80-83sc had the dual vacuum unit. As long as the hose is hooked in to port vaccum you should have no problem. Again anyone please correct me or add to this if its incorrect in anyway...TargaEuro

If you read tbitz's post above, he reports that when he applies a vacuum to his distributor, the RPM's go up. Does this indicate a vacuum advance or vacuum retard? If I take the cap off of my dizzy, and apply a vacuum, which way should the rotor move? Clock wise for advance, or CCW for retard?

Does anybody have a good link to something that explains the differences between a vacuum retard and vacuum advance system?

I'm beginning to feel like a vacuum retard myself.



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Bill Krause
'79 911SC Euro

I do believe that the vacuum hose should connect below the throttle plate...I really cant get a clear pic though. The hose on my car was removed and plugged some time ago for some reason. My car runs good without the vacuum retard hooked up other than the surge I was explaining earlier. I am Installing a new mechanism since mine wont hold vacuum anymore and hooking it up too. If its on the car I think that it should work.

Im just going by what the factory workshop manuals say. Mine wont hold vacuum so I dont know what the hell its going to do when I get the new mechanism installed..So much for those expensive Factory coloring books I bought...

Where's John?....Help us please.....

From the little research I've done, a vacuum retard system is used to reduce emissions at idle, so plugging this up won't hurt performance. The connection to a vacuum retard system is to the bottom side of the throttle body, intake valve side, where there is a vacuum at idle.

The vacuum advance is used to advance timing when the throttle opens, and before the centrifical advance starts to come into play (lower RPM's). It's connection is to the top of the throttle body so it only sees a vacuum when the throttle is opened.

All of the diagrams that I can find for a '79 SC show the vacuum hose going to the top of the throttle body, but I can'tfind anything that says my dizzy has a vacuum advance.

Please correct me if I am wrong or confused!!!

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Bill Krause
'79 911SC Euro

Bill, That sounds logical to me. From what I read vacuum advance came in 1980-83 models. Lets see what others think or know...TargaEuro

[This message has been edited by TargaEuro (edited 04-02-2001).]

The 80-83's had both vacuum advance and vacuum retard. They have two vacuum hoses running to the top and the bottom of the throttle body.

I pulled my vacuum line off and applied a vacuum to my dizzy. It didn't change idle speed at all. I then shut the car off and pulled off the cap. The mechanical advance seemed to work, but applying vacuum didn't move the rotor. Now I'm hoping it is vacuum retard, cause I don't care if it doesn't work. But if it's an advance system, then I need to get it fixed.



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Bill Krause
'79 911SC Euro