I asked this question a few months ago at the end of a long thread, but I don't *think* it was ever answered. So I thought I'd throw it up here again. I was told (by Damon, I think it was) that my vac. adv. has to be set so that the amount of vacuum it takes to pull the arm "all-in" has to be lower than the amount of vacuum I have at idle, in gear. And he said if i can't do this then I'd have to use the timed port, instead of full manifold vac. My question is, if I have to use the timed port, then what do I set the vac. adv. to?

So... full port = vac. adv. "all-in" adjusted slightly less than idle vacuum.

timed port = vac. adv. "all-in" adjusted ...??

The timed port doesn't give you any vacuum at idle. It is ported above the throttle blades. Therefore the vacuum advance wouldn't be activated at idle.

Shawn, does the vacuum can have an issue right now? If you don't know do this.

Hook the vac to manifold and set idle speed.
Point timing light at balancer
Does timing vary?
If no, have a beer and do a burnout.
If yes, vac can could be an issue and repost.

No the can is fine, so I had a beer;) but for someone who had to use the timed port....I was assuming the vac can needs adjusted to a certain spec even when hooked up to a the timed port. Because the time port does have vacuum off idle which is advancing the can right?
The timed port doesn't give you any vacuum at idle. It is ported above the throttle blades. Therefore the vacuum advance wouldn't be activated at idle. but what about off idle? Doesn't the can need adjusted for optimal results when the vacuum kicks in off idle with the timed port?



By the way Rick, how are those Badgers gonna do this year.... Lord knows it won't be the Buckeyes winning the Big 10 this year....or will it?! ;)

Man, once Ron Dayne left and you could not drink anymore I gave up on the Badgers. Sort of. The paper though has been very quite all summer in the sports section so I think I'll root for Northwestern. See if a bunch of engineering majors can take the trophy.

On the vac can, unless the cruise vac is whack as compared to the vac can requirements it doesn't matter. On most vehicles engines like around 8-10 inches of vacuum advance under cruise conditions. Once the vacuum is down around 5 or so the vac advance shoudl basically be pulled out.

I asked this question a few months ago at the end of a long thread, but I don't *think* it was ever answered. So I thought I'd throw it up here again. I was told (by Damon, I think it was) that my vac. adv. has to be set so that the amount of vacuum it takes to pull the arm "all-in" has to be lower than the amount of vacuum I have at idle, in gear. And he said if i can't do this then I'd have to use the timed port, instead of full manifold vac. My question is, if I have to use the timed port, then what do I set the vac. adv. to?

So... full port = vac. adv. "all-in" adjusted slightly less than idle vacuum.

timed port = vac. adv. "all-in" adjusted ...??

I will give it a shot . The Vac needs to be all in at idle on a carb base port .Not manifold You can`t adjust it to dump that fast from a manifold port . Also connecting the adv vacuum to manifold will cause surging & exhaust backfire at low speed . Excessive cam will starve advance vac & cause eratic idle.. Sounds like you need a vac pump if vac reads under 10 at idle Or go straight mechanical & not worry about vacume .

so,, from rick's test that you did,, it's not varying the rpms arround and the can is all in,,,,,,,below the idle vacuum at idle. you still may have to make a plate to limit the total vacuum advance if it pings,, or mess with the spring tension to control the rate it comes in at,,,,,if it's an adjustable vacuum can.

both vacuum sources will pull vacuum just off idle. the timed port is pulling venturi vacuum,, which is airspeed past the ported hole in the venturi above the butterfly's as stated above by zach. the manifold source'd port is generally more load compensating,, and forgiving in most cases allowing less initial timing settings for easier cranking. then the vacuum brings idle timing up to a reasonable range.

on the average,, an engine will like somewhere between a general area of 20*-25* of total timing (all types together,, initial,,vacuum,, and idle/mechanical) at idle speeds. some lower cylinder pressure setups can tolerate more initial timing and on tip-in,, and some with more cylinder pressure can take less. general ranges there. each setup is a tad bit different. hope it helps. andy

RickWI and everyone...

thanks for the replys.... still a little confusing but I'll manage. hell full throttle and I don't worry about a adv. can anyway right?;)


Rick, just as long as you ain't rooting for those damn Wolverines......
I'm a damn Buckeye freak..... can't wait 'til football season..... thats when things get crowded..... football games and car shows all in the same day.

hell full throttle and I don't worry about a adv. can anyway right?

pretty much so. the carb would have to be REALLY REALLY small for the application to start pulling the intake manifold back into 5"-7" vacuum at top rpms. but it is possible in that scenerio to have it add timing back in from the vacuum can in that situation. not the norm though. andy

Bahaha, nope lived in MI for two years and even then I wasn't a MI fan. I have to say it's always a sweet victory when the Badgers beat Michigan.