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Too Much Vacuum Advance

30K views 10 replies 7 participants last post by  LATECH 
#1 ·
I found my vacuum advance is way too much. My base is set at 16, and plugging in the vacuum advance takes it up to 45-46 at idle. So I'm going to get an adjustable canister, just wondering why the old one would do this, maybe a tear in the diaphragm?
 
#5 ·
I presume you meant XE268 cam rather than 368.

If the idle doesn't expose the timed port by lifting the throttle blades above it, I'd suggest connecting the vacuum advance to that. I think you, also, should check the centrifugal to be sure that it isn't starting to act at 900 RPM shouldn't be a problem but these are things that bear checking to be sure. What keys this in my mind is that most vaccum advances only have from 7 to 14 degrees in them so with 18 to start with I'd expect something like 25 to 32 degrees not 45-46 degrees. Using a full time port as you are can be a tool to pull the base back making starting the engine easier on the starter motor, then when it fires the vacuum advance reading the intake directly will pull up the rest of the advance that makes the idle speed and a smooth transition to higher RPMs. An adjustable vacuum can is useful in this regard, the factory units come in different ranges but are not adjustable.


But keep in mind that more aggressive cams take some fiddling at low RPMs they do mess with the mixture and volumetric efficiency both of these play into the compresson ratio. If the compression is on the low side the engine will want more advance than usually expected. So all of this takes some tinkering to dial in. The big thing is staying under the detonation threshold, what it does under that point is pretty open to whatever advance works the best.

Bogie
 
#7 ·
88 >> The crane limiter plate desribed in the previous post link/pic is the best way to take care of the HEI limit, at the highest/widest part of the plate lobe gives about 10-12 deg crank advance. The plate shortens the travel of the canister rod connected to the reluctor on a HEI dist. Adjusting the canister set screw does not shorten the amount the rod travels. (exception below)

Although Accell claims that on their ajustable unit the screw does change both the range and the total amount, this is un tested in my world. I have only monky'd with the crane parts, and I can tell you the crane adjustable kit with springs, canister and limtier plate works.

Most adjustable canisters will not limit the amount of vacuum advance applied, they only allow the tip in point to be changed, the effect is the same amount of advance added but at a different starting point in engine vacuum. A typical HEI canister adds about 18 deg of crank advance by 14 inches vacuum or above, this is too much advance.

For the most part if the engine pulls over 13 or 14 inches the adjustable canister will add maximum advance no matter what the tip in point is set to (allen set screw down in canister hose nipple fitting). This is why you need the limiter plate.

Idle Exception >>> If you are running a big cam engine making low idle vacuum (< 13 ") then the adjustable canister will in effect limit the maximum amount by turning the set screw because the 8 -12" hg vacuum at idle is within the range of the spring. This all goes out the window when the motor pulls 20 inches under decelleration, down shifting etc.
 

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#8 ·
Oldbogie, I tried running it on timed port but couldn't get the engine to run smooth to save my life and wouldn't even attempt to idle under 1000rpm. I have the timing set at 16 because I brought it up to 3000rpm and set total to about 36 and that's where it fell when I brought it back down to idle. It seems to be happiest there so far.

Crankshaftcoalition is always a good read!! I have the Accel canister on the way to my house tomorrow, so we will see how that goes.

Does anyone have any idea what would be causing 30 degrees of advance from the stock canister I have on there now?
 
#9 · (Edited)
The advance canister should have a number stamped on the bracket. There are many different amounts of advance available with stock canisters as well as vacuum levels that they operate at.

The most I've seen is 24 degrees. That doesn't mean that a 30 degree canister wasn't available.

Check the numbers. It could be that tolerances got off on the piece you have. The amount of advance in a stock unit is determined by the slot that the operating rod moves in. Some adjustable units have a lot of advance (30 degrees or more) and come with a notched plate that you use to simply limit travel.
 
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