SpyderD

Found something curious yesterday. I'm trying to raise the idle a bit, it idles at about 500-550 now. When I raise the idle speed I noticed I get to a certain point and the idle jumps up to 2000 or so - no middle ground. The best I can get is 500-550.

Now I have the vacuum advance connected to the ported vacuum on the carb (NOT the manifold vacuum) so I confirmed my suspicion with the vaccum guage - I have vacuum from the ported valve, when I increase the idle speed to a certain point it raises the vacuum to pull from the vacuum advance.

I have a very large cam (comp cam 280) so the vacuum is only at 8 at the manifold valve and 5 at the ported valve. My understanding of what is going on is that I raise the RPM via the idle speed screw it increases the vacuum to the ported valve, in turn pulling from the advance when it gets to a certain point (it seems to be pulling from the advance at 6 lbs of so), in turn increasing the timing, RPM, and vacuum even more, and again in turn pulling more and more from the advance until it gets maximum vacuum advance in - thus no middle ground, just a choice of no vaccum advance at idle or max vaccum advance at idle.

I know I can fix it by correcting the initial timing, or adjusting the vaccum advance, which I will do soon. Or I can disconnect the vacuum advance hose (which I do not want to do). Right now the 500 RPM idle isn't bad, I can live with it, it never stall at lights, but I think it will be happier idling at 650 or 700. But is it normal to get a vacuum at the ported valve? The carb is a holley 650 mechanical secondary.

Fevre

This article should help:

http://forums.corvetteforum.com/zerothread?id=680541

SpyderD

Good article, I've actually read it before. My car has no stumble or hesitation at all and does not run excessively rich as far as I know. The article references a 750 carb, I have a 650. The previous 750 double pumper carb I had DID have this problem.

I was only curious as to why I have a vacuum at the idle port, the article really doesn't explain. Is that because the idle speed screw or idle mix screws may be opened up to far?

Fevre

Would think it comes when you adj you idle up higher it exposes the transfer slots and opens the ported vac source. I had a problem with some passeges being plugged/blocked in my 750 dp and I had to open up the throttle plate a ways to get the correct idle. Once I cleaned and blew them out with compressed air it helped.


[Modified by Fevre, 11:14 AM 11/3/2003]