Vaccum Advance
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Vaccum Advance
Can someone please explain to me what the adjustments are for on a vaccum advance? It is for a stock 2bbl motorcraft carb and OEM distributor 351M 78. The outer canister moves left to right and I am not sure what that would be advancing or retarding,I would imagine, but not sure what that would help me gain or fix what problem. If it is adjusted would other things need to be adjusted to match IE carb, timing?
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From: Big Sky Country
Turning the distributor clockwise will advance the initial timing.
Turning the distributor counter clockwise will retard the initial timing.
If your vacuum canister is the adjustable type (using a 3/32" allen wrench inserted into the nipple) and turning clockwise will increase vacuum advance...
Turning counter clockwise will decrease vacuum advance.
Turning the distributor counter clockwise will retard the initial timing.
If your vacuum canister is the adjustable type (using a 3/32" allen wrench inserted into the nipple) and turning clockwise will increase vacuum advance...
Turning counter clockwise will decrease vacuum advance.
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Yea the crazy part is that without an allen wrench and just turning the "outside" of the cannister there is a "noticeble" difference in the way the truck runs and drives. The cannister is an octagon shape, as in the shape of a bolt, and if you turn it left or right there seems to be some sort of adjustment going on. The actual adjustment of the distributor isn't as big of an issue as the vac advance seems to be, the only thing with the distributor is not knowing if vaccum advance is moved does the timing need to be adjusted at the same time, or vice versa, do they work together or individually?
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Thanks for the pic that at least gives me a visual of what I'm dealing with , but what are the adjustments, what am I doing by going say either clockwise or counter clockwise? I think the question would be when I turn the cannister clockwise what am I doing and why would I do it? What problem or issue am I trying to resolve?
Post Fiend
Joined: Apr 2008
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From: Big Sky Country
Tuning vacuum advance should be done only after total timing is correctly dialed in...
Total timing is initial advance + mechanical advance = total advance (vacuum advance does not factor into total timing).
Vacuum advance cannot be tuned with a timing light in your driveway, the engine needs to be under load, it's a process of trial and error.
Instead of trying to explain it all here I suggest reading this excellent article on the subject...
FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power
Total timing is initial advance + mechanical advance = total advance (vacuum advance does not factor into total timing).
Vacuum advance cannot be tuned with a timing light in your driveway, the engine needs to be under load, it's a process of trial and error.
Instead of trying to explain it all here I suggest reading this excellent article on the subject...
FORDMUSCLE webmagazine: Timing is Everything - Distributor Curving for Maximum Power
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