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Cfox
07-08-2013, 05:51 PM
I'm trying to hook up my vacuum advance on my 750 Holley.
I have adjusted the idle screw to where it will not back off any more and I still have full vacuum from the ported port. I adjusted the idle mixture to keep it running with it all the way backed off. It's on a 1987 305 H.O. in my Trans Am.

What would cause this and has anyone had this same problem?
This port is supposed to be 0 vacuum at idle and increase as you open the throttle (venturi vacuum) to advance the timing on the HEI distributor.
I have adjusted the metering block at idle and have tried several different adjustments and I can not get the vacuum to even slow down on this port. This is the port on the pass side of the metering block just above the mixture screw.

What would cause this and how can I make this work for vacuum advance? (Without replacing it?)
The metering block has new seals and gaskets. I replaced them when I started jetting this carb and rebuilt it for this old Trans AM.
When I went through it I did not see anything abnormal about any of it. I have rebuilt countless carbs. I'm not a rookie but this one has me buffaloed!
The carb is about 25-30 years old and has been sitting on a shelf in my shop for years. So it's not wore out! Everything is tight and runs good other than the constant vacuum issue on the ported vacuum port.

Thanks in advance for your help. cfox

Danny Cabral
07-09-2013, 03:30 AM
What would cause this and how can I make this work for vacuum advance?
I'm guessing the carburetor has the wrong metering block gaskets installed.

Vacuum advance should be connected to full manifold vacuum; not ported.
8-12 of vacuum advance is the typical limit (some canisters are adjustable).
The (ported) timed spark port is only for emissions/EGR engines.
Ported/timed vs full manifold vacuum advance:
http://www.chevytalk.org/fusionbb/showpost.php?post/1905429/

Cfox
07-09-2013, 03:22 PM
When I have the vacuum advance hooked up to manifold vacuum it fully advances at idle.
This causes detonation when under load because of too much timing and lower and mid range RPMs.

For now I'm running a stock 305 H.O. with stock cam.
Any ideas?

Danny Cabral
07-09-2013, 05:06 PM
When I have the vacuum advance hooked up to manifold vacuum it fully advances at idle.
It's only supposed to advance the timing 8-12. What's the timing at idle with the vacuum advance connected to full manifold vacuum?


This causes detonation when under load because of too much timing and lower and mid range RPMs.
That's because you don't have a custom timing advance curve for your engine. But I understand, this is now a lost art.
"Under load", there wouldn't be any difference. The difference between ported and full manifold vacuum only occurs at idle.
A proper timing advance curve uses RPM (centrifugal weights, springs & limiter) & load (vacuum canister) to change the timing.
Intake manifold vacuum (kPa) is a more accurate representation of engine load, as opposed to ported.
An EFI Timing Table doesn't use throttle position or ported vacuum; only RPM & intake manifold vacuum.