timing
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
Quick primer...
"Total timing" is initial advance plus mechanical advance, period. Vacuum advance does not factor into total timing, i'll explain why later.
Sometimes various terms get confused with one another....initial/base advance are the same thing, same is true for mechanical/centrifugal advance, they're the same thing, these two (initial and mechanical) when added together equal your total timing, generally around 34-38 degrees.
This is where most of the confusion/misinformation begins.....
.....Vacuum advance, when connected correctly, i.e. ported/timed, is zero at idle, and also zero at wide open throttle.....vacuum advance comes on heaviest just off-idle, stays steady at part-throttle, and completely vanishes at WOT, so therefore it's not factored into the "total timing" equation. (doesn't mean vacuum advance is not important, because it is, but that's a whole nother story).
Hope this clears it up a lil bit.
"Total timing" is initial advance plus mechanical advance, period. Vacuum advance does not factor into total timing, i'll explain why later.
Sometimes various terms get confused with one another....initial/base advance are the same thing, same is true for mechanical/centrifugal advance, they're the same thing, these two (initial and mechanical) when added together equal your total timing, generally around 34-38 degrees.
This is where most of the confusion/misinformation begins.....
.....Vacuum advance, when connected correctly, i.e. ported/timed, is zero at idle, and also zero at wide open throttle.....vacuum advance comes on heaviest just off-idle, stays steady at part-throttle, and completely vanishes at WOT, so therefore it's not factored into the "total timing" equation. (doesn't mean vacuum advance is not important, because it is, but that's a whole nother story).
Hope this clears it up a lil bit.
Senior User
Joined: Mar 2004
Posts: 280
Likes: 2
From: Alabama
Save the old ones
Joined: Dec 2002
Posts: 666
Likes: 4
From: Boyne Mich
Montana has it right.
I just went through this as well.
Don't foget to disconnect the vacuum advance and plug the line when setting initial timing.
That messed with me a bit with a high idle carb and and plugged into manifold vacuum and not the ported vacuum.
I just went through this as well.
Don't foget to disconnect the vacuum advance and plug the line when setting initial timing.
That messed with me a bit with a high idle carb and and plugged into manifold vacuum and not the ported vacuum.
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Senior User
Joined: Oct 2012
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i think i understand the initial and mechanical timing part but its the vaccum part that confuses the hell outta me. i just dont see how its not part of total timing. i thought initial advance + mechanicall advance + vaccum advance = total advance??? i guess i just dont see how its separate from the other advances, so your saying just to ignore vacuum advance? ugh i gotta go back to work but this still baffles me 
Laughing Gas
Joined: Aug 2000
Posts: 1,005
Likes: 63
From: Eastern, Idaho
Read the link posted earlier. Vacuum advance is directly related to the position of your foot to put it simply so it can vary alot depending on engine load. When the throttle is opened vacuum drops. So you will have little vacuum advance when flooring the pedal. But when cruising on the highway at part throttle the vacuum comes back up and you will get more advance from vacuum to help with mileage.
Post Fiend
Joined: Apr 2008
Posts: 8,261
Likes: 15
From: Big Sky Country
I don't know how to make it any more clear than I already have....
Initial advance is static, you set it and forget it, it doesn't change.
Mechanical advance is just that, mechanical, it advances as the rpm's increase.
Vacuum advance is a load sensing device, think of it as a switch, OFF when the engine doesn't need it (idle and WOT), and ON when the engine does need it (part-throttle cruise).
Once again, this is why vacuum advance is not factored into total timing.
Initial advance is static, you set it and forget it, it doesn't change.
Mechanical advance is just that, mechanical, it advances as the rpm's increase.
Vacuum advance is a load sensing device, think of it as a switch, OFF when the engine doesn't need it (idle and WOT), and ON when the engine does need it (part-throttle cruise).
Once again, this is why vacuum advance is not factored into total timing.
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Senior User
Joined: Oct 2012
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thanx for the explanation i think i understand it now. so if i have this right there is no vaccum at higher speeds or higher rpms and this is why vaccum advance is not part of total timing. vaccum advance only kicks in when your cruising with part throttle. do i have that right? 
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