Please explain what happens to the timing when using manifold vacuum when you stomp on the gas off idle.
I know, it retards. do you think that is good or bad?
Vacuum advance issue
- Thread starter Cam1399
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I know, it retards. do you think that is good or bad?
I agree. It's all about what a particular combo responds to. I try not to box myself in by old hot rod adages. I'll try it and if it doesn't work, then I scrap it.
Does Wizard set his timing in neutral or in gear?
Dan Georgino
Well-Known Member
Oops, that was an accident, pressed the wrong one. As for full vacuum at idle, didn't say I do it, or any street driven car did. Learn the word ported vacuum, try to keep up, no wonder you don't like UTG, you can't follow him to well.
Dan Georgino
Well-Known Member
Nope, still give it a X.
Dan Georgino
Well-Known Member
You said Big red X, it's the same size as the thumbs up! Lol
Dan Georgino
Well-Known Member
Yes, I know. You should be asking the guys that use it that way. Sometimes they have a big cam that has a very low vacuum at idle, and they use it this way. Ask them.
Dan Georgino
Well-Known Member
Also remember, vacuum advance has nothing to do with performance.
I don't need to ask, it's a crutch.
The engine will run much cleaner and require less throttle at cruise.
GM used manifold vacuum to the advance can on several models through the years.
Dan Georgino
Well-Known Member
There you go, now you get it. Cleaner? Who cares!
Dave999
Well-Known Member
In my opionon
the initial advance and vaccum can have been set to facilitate two "fixes" in the past, and those two solutions are not compatible with each other.
1) Today :- enough initial timing to mask the fact that the vacuum can is now disconnected.
2) previously, back in history:- the inital timing was set as per the book nominally 5BTDC, and the vacuum can has been adjusted so that it could be plugged into manifold vacuum port to pull in a lot of advance at idle, to mask the fact that the mechancial advance book setting wasn't adequte for the state of tune or fuel grade. you would get very little vaccum advance from then on up the rpm range, but maximum vaccum advance at idle and just off.
Mechanical advance was still set at book setting from 196?...and that setting is no good on todays fuel , you need 10+*
i.e vaccum advance was used for easy starting idle and just off idle operation
rather than its orginal use on ported vacuum for more advance in the just off idle rpm range
and high speed throttle not quite closed cruise.
hence OP now has 2 parts of 2 solutions in place to mask what was probably sluggish performance off idle or a bloody big flat spot.
as suggested by all/many
wind the vaccum can adjuster totally the other way and start again
and suggested by me
take 1/2 the inital mechanical timing out and start again
increase initial mechinical timing until you get no smell of fuel off the tail pipe at idle with a hot engine, at a guess somewhere betwee 8 and 15*
plug vaccum can in to ported (upper carb vacuum pipe) vaccum
reset idle mixture
reset idle speed to something that means it doesn't stall as you move it into drive from neutral 800-1000 rpm
and see how ya go when driving
if the vacuum can is set right lifting your foot off the gas should result in a feeling that the car will go on and on and on...like a cart on rails, little noticeable G force applied to you to indicate an abrupt slow down once you lift off the gas. try adding a little more until you get to this point, expect it to ping under load if you go too far.
there will be other ways to get it right but sometimes starting again works....
cam cr porting headers all play their part and it will become aparent if your igntion set up can not cater for any modifications to the engine you have made.
dave
the initial advance and vaccum can have been set to facilitate two "fixes" in the past, and those two solutions are not compatible with each other.
1) Today :- enough initial timing to mask the fact that the vacuum can is now disconnected.
2) previously, back in history:- the inital timing was set as per the book nominally 5BTDC, and the vacuum can has been adjusted so that it could be plugged into manifold vacuum port to pull in a lot of advance at idle, to mask the fact that the mechancial advance book setting wasn't adequte for the state of tune or fuel grade. you would get very little vaccum advance from then on up the rpm range, but maximum vaccum advance at idle and just off.
Mechanical advance was still set at book setting from 196?...and that setting is no good on todays fuel , you need 10+*
i.e vaccum advance was used for easy starting idle and just off idle operation
rather than its orginal use on ported vacuum for more advance in the just off idle rpm range
and high speed throttle not quite closed cruise.
hence OP now has 2 parts of 2 solutions in place to mask what was probably sluggish performance off idle or a bloody big flat spot.
as suggested by all/many
wind the vaccum can adjuster totally the other way and start again
and suggested by me
take 1/2 the inital mechanical timing out and start again
increase initial mechinical timing until you get no smell of fuel off the tail pipe at idle with a hot engine, at a guess somewhere betwee 8 and 15*
plug vaccum can in to ported (upper carb vacuum pipe) vaccum
reset idle mixture
reset idle speed to something that means it doesn't stall as you move it into drive from neutral 800-1000 rpm
and see how ya go when driving
if the vacuum can is set right lifting your foot off the gas should result in a feeling that the car will go on and on and on...like a cart on rails, little noticeable G force applied to you to indicate an abrupt slow down once you lift off the gas. try adding a little more until you get to this point, expect it to ping under load if you go too far.
there will be other ways to get it right but sometimes starting again works....
cam cr porting headers all play their part and it will become aparent if your igntion set up can not cater for any modifications to the engine you have made.
dave
Dan Georgino
Well-Known Member
I don't really care what anyone says or recommends. This is real easy yall. Run it where it responds best. Ported, manifold or not at all. Whatever the engine runs best with is what it needs.
Big cam, inconsistent vacuum, the very reason not to run full vacuum advance at idle.
Initial timing is crucial when tuning a carburetor during idle and transition.
Again, what happens when you mash the pedal, all the vacuum advance, say 10 degs goes a away.
Dan Georgino
Well-Known Member
Yes, I know. Like I said before, ask the one that use intake manifold vacuum.
All I can say is, Vixen's engine likes ported best and I've tried manifold, ported and leaving it plugged. The cam is 250@.050 on a 108 so that qualifies as a "big cam" for a slant 6. It only has 6hg at idle.
Ok, we're good.
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