I have a 90 roller motor set up, but with carb and points dist(autolite, single vacuum advance). I played with the timing to get it to both idle fine and hold revs(in neutral) well. I had it set at 10 degrees btdc which idle'd ok but ran like crap with any throttle, so I backed it down closer to 0(guess) However, I cannot use the vacuum advance. When I hook it to the timed port, it idles fine, but when you throttle it it seems it overadvances and starts 'missing'(shaking). If I hook it to manifold vacuum it just dies immediately. The vacuum canister does hold vacuum, so I don't think it's a leak. So, which port do I use(ported or full vacuum), and what do you suggest the problems could be? Do you always set the timing with the advance unplugged?
vacuum advance help
- Thread starter woedend
- Start date
rd
Founding Member
Points? They might be part of the problem...
I would pull no 1 plug, set the engine up on no 1 at tdc, make sure the marks line up ok. Then, set the base timing at 6-8, with the vac advance disconnected.
With a timing light, and the vac unhooked and the source plugged, make sure the mech advance is working.
If this is a stick car, it may drive ok with just the mech advance. An auto trans car needs vac and mech or a very loose converter.
I have the vacuum advance connected to the venturi spark port on a Holley 600 vac sec on my street car. Works fine. You should have no vacuum advance at idle, and when you have higher airflow throught the carb, you should have more advance. If you go to manifold vac, you will have max vac advance at idle, not what you want. my opinion....
I would pull no 1 plug, set the engine up on no 1 at tdc, make sure the marks line up ok. Then, set the base timing at 6-8, with the vac advance disconnected.
With a timing light, and the vac unhooked and the source plugged, make sure the mech advance is working.
If this is a stick car, it may drive ok with just the mech advance. An auto trans car needs vac and mech or a very loose converter.
I have the vacuum advance connected to the venturi spark port on a Holley 600 vac sec on my street car. Works fine. You should have no vacuum advance at idle, and when you have higher airflow throught the carb, you should have more advance. If you go to manifold vac, you will have max vac advance at idle, not what you want. my opinion....
Your TOTAL advance should probably be limited to about 30 degrees. That's mechanical plus vacuum. Check the mechanical with a timing light at about 2500-2800 rpm (whatever it reads above base timing). If you put manifold vacuum to the advance to test it and it stalls it, there is an allen wrench hole in the center of the advance you can turn to adjust the total vacuum advance. Just insert the allen wrench (1/8", I think) into the port where the vacuum hose conects and start with 2 turns clockwise. Play with it until you get it to perform correctly to your liking. The advance should go to the ported (venturi) vacuum port for normal operation.
Michael Yount
Mustang Master
I think you'll be much happier with a recurved dist. and completely do away with the vacuum advance. I've done this on most of my cars with mech. advance (been a LONG time since....).
Your car will probably like something on the order of 34-36 degrees total all in by about 2800-3000 rpm. Do away with the vacuum advance. Set the intial around 12-18 depending on the what the engine likes (I'd try 16); then set your centrifugal weight stop so that you're adding an additional 18 degrees for a total of 34. Pick light enough springs so that all the centrifugal is in by 2800 - just try different springs and use your timing light with rpm to see when the total is all in.
Won't have a chance in hell of passing emissions - but it should run great, have good throttle response.
Your car will probably like something on the order of 34-36 degrees total all in by about 2800-3000 rpm. Do away with the vacuum advance. Set the intial around 12-18 depending on the what the engine likes (I'd try 16); then set your centrifugal weight stop so that you're adding an additional 18 degrees for a total of 34. Pick light enough springs so that all the centrifugal is in by 2800 - just try different springs and use your timing light with rpm to see when the total is all in.
Won't have a chance in hell of passing emissions - but it should run great, have good throttle response.
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