Vacuum advance maxed out at idle?

FULL-THROTTLE

New Member
Mar 9, 2015
4
ma
Hey good afternoon everybody I have a small block Chevy with a summit aluminum distributor with vacuum advance and when the car is at idle roughly 800 RPMs the vacuum maxes out and stays maxed out I do not have a vacuum leak or anything I just don’t understand what the heck is going on
 

Gary S

Administrator
Lifetime Gold Member
Apr 14, 1999
24,585
Bismarck, North Dakota
There are two kinds of ports for vacuum on a carb. Some ports have full time vacuum, and others are ported to provice vacuum only when the throttle plates are open.
Often, the car manufacturers used full time vacuum for street driven vehicles.
 

pomartin

Veteran Member
Sep 16, 2019
279
Full VA at idle is the only logical setup, so you are on right way :)
It will allow you to lean out the idle mixture and increase efficiency at low piston speed. Mixture that is not mixed good (low rpm) burns slowly, meaning it takes more time to burn. Activated VA at idle means you ignite this slowly burning mixture sooner giving it time to burn fully in cylinder.
In case of no VA at idle that mixture would burn also in exhaust, manufacturers used this effect to reduce emmisions, but it hurts performance.

It is normal to see about 35-40 degrees of advnce at idle speed.
 
Last edited:

Twisted_Metal

Administrator
Staff member
Lifetime Gold Member
Feb 26, 2004
31,460
Bloomington, MN
Exactly.

Your vacuum is highest at idle because the throttle blades aren't open and that will pull the full advance on the dsitributor.
(Assuming you have the vacuum advance connected to manifold vacuum, rather than ported vacuum.)
 

2ndGenCrazy

Veteran Member
Aug 18, 2011
893
Upstate New York
Full vacuum advance while engine is at idle speed and throttle against the idle set screw (closed) is correct.
That being said, a cam with high valve overlap can result in idle vacuum low enough that vacuum advance no longer functions as intended.
In that case, many people use ported to stabilize spark timing and therefore idle speed. You lose combustion efficiency when you don't have enough timing in at idle, but that's the price to pay for running that cam profile. This gets especially bad when the cam is not matched with the compression ratio it was designed for.
 

kenny77

Veteran Member
Lifetime Gold Member
Jul 31, 2001
3,319
Ft. Lauderdale, FL
Use FULL manifold vacuum and use an adjustable vacuum advance can . Zero in on your idle by adjusting idle speed and by adjusting the amount of vacuum advance for the best idle.

I usually use a vacuum advance limiter plate too to limit that amount also.
 

pomartin

Veteran Member
Sep 16, 2019
279
That is way too much advance at idle .
You are right, I made a mistake there.
Usually HEI have about 20 VA advance, meaning full idle timing would ve about 35 degrees... so 35-40 would be a better statement.
I have 41° at idle myself (22 initial and 19 VA)
 

frank5s

Veteran Member
Sep 6, 2008
315
Naperville Il

On the link, you can see what your distributor spec were and adjust to your liking
Having install vortec heads on an 1980 Z , l bumped my initial timing to 10 degrees before
with 22 degrees curve in the distributor all in by 1500 rpm. the vacuum advance canister adds
18 at idle. So cruising l have 50 degrees total . 10+22+18. Motor idles smooth , runs cool and what a different the vortec heads made.
Here is a link all about Vac advance canister. The man who wrote this is very well respected
in his knowledge about G.M. motor

 




Top