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Can I please get a definitive answer on this?

JG1966

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When you adjust a vacuum advance canister, are you adjusting the amount of timing it puts in the engine OR the amount of vacuum needed to accuate the advance?
Four people have told me one way, three the other.
If it matters, it's a JEGS Mopar Electronic ignition distributor on a 340. Thanks
 
This is from Summits site. I take no credit.

The vacuum advance is an Ignition Timing control mechanism built into some distributors. It is made up of:

  • A vacuum canister,
  • A linkage, and
  • An advance plate attached to the trigger mechanism inside the distributor.
How does it work?
  1. At part throttle, the throttle blade(s) are only part way open.
    1. This creates increasing vacuum in the intake manifold.
  2. As manifold vacuum rises, it pulls on the diaphragm inside the canister.
    1. The diaphragm is connected to the advance plate by a linkage.
  3. The movement of the linkage rotates the trigger mechanism.
  4. This causes the ignition to be triggered sooner (advanced).
Can I adjust it?
Some vacuum advance mechanisms can be adjusted.

If you hear an Enigne Knock at cruising speed, you may need to adjust your vacuum advance mechanism.

How does it affect performance?
Under a light load and part throttle conditions, timing can be advanced. This improves throttle response and makes the engine more efficient. It also helps the engine run cooler. The vacuum advance provides this benefit BEFORE the Mechanical Advance provides Total Timing.

Vacuum advance distributors are best for street vehicles. The vacuum advance feature has NO EFFECT on full-throttle power. They provide no benefit for engines that spend most of their life at high rpm (race engines).

A vacuum advance unit can add up to 15 degrees of advance. The amount of advance depends on how much vacuum is applied.
 
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When you adjust a vacuum advance canister, are you adjusting the amount of timing it puts in the engine OR the amount of vacuum needed to accuate the advance?
Four people have told me one way, three the other.
If it matters, it's a JEGS Mopar Electronic ignition distributor on a 340. Thanks

The allen screw changes tension on the spring so it changes the amount of vacuum required.... To limit actual degrees of vacuum advance you need to alter the stop built into the arm sticking out of the vacuum advance... Some guys have actually drilled & tapped a hole in the distributor housing so a screw can be used to limit movement of the breaker plate like shown above..
 
if your at wide open throttle you dont need vacuum advance.
Thanks Snake. :). I've always run with it plugged but I'm going to give it a try and see how she likes it. And to keep this thread going, I'm using manifold vacuum not ported. :)
 
Ok. Yes the screw adjustment 'starts' when the vacuum can kicks in. However: It does, in a sense, ALSO limit the amount of timing added, because the can adds advance over a range of vacuum. Meaning, if your can is adjusted to start adding advance at say 15", the spring does not slam the diaphragm into full effect at that point. As the vacuum increases, the amount of advance will increase, to a point.
I encourage everyone--hook a vacuum gauge up to manifold vacuum, to where you can see it while driving, and go drive and note the readings at different speeds/throttle conditions.
Then go home and hook up a vacuum tester like a mity-vac to the distributor can, with the cap off so you can watch the advance plate's movement. You'll see exactly how and at what vacuum reading the can responds as vacuum increases.
Typically, they'll continue moving the plate over about a 10" vacuum range(depending on the can used) so for example if you set the can to activate at 15", and the most your engine makes is say 18", it won't add as much timing as it would if you set it to activate at 10". So if you know how your engine acts you can control the 'how much' by controlling the 'when'. (Of course, seeing the actual # of degrees added must be done with your timing light).
On mine, out of the box the vacuum advanced my timing to over 60° because it started too soon. I adjusted it so that my overall total is right about 50° which the engine seems happy with. Whew! Long-winded sorry but I hope that helps.
*FYI cans can be modified to provide a greater or shorter duration of diaphragm operation as well
 
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Ok. Yes the screw adjustment 'starts' when the vacuum can kicks in. However: It does, in a sense, ALSO limit the amount of timing added, because the can adds advance over a range of vacuum. Meaning, if your can is adjusted to start adding advance at say 15", the spring does not slam the diaphragm into full effect at that point. As the vacuum increases, the amount of advance will increase, to a point.
I encourage everyone--hook a vacuum gauge up to manifold vacuum, to where you can see it while driving, and go drive and note the readings at different speeds/throttle conditions.
Then go home and hook up a vacuum tester like a mity-vac to the distributor can, with the cap off so you can watch the advance plate's movement. You'll see exactly how and at what vacuum reading the can responds as vacuum increases.
Typically, they'll continue moving the plate over about a 10" vacuum range(depending on the can used) so for example if you set the can to activate at 15", and the most your engine makes is say 18", it won't add as much timing as it would if you set it to activate at 10". So if you know how your engine acts you can control the 'how much' by controlling the 'when'. (Of course, seeing the actual # of degrees added must be done with your timing light).
On mine, out of the box the vacuum advanced my timing to over 60° because it started too soon. I adjusted it so that my overall total is right about 50° which the engine seems happy with. Whew! Long-winded sorry but I hope that helps.
*FYI cans can be modified to provide a greater or shorter duration of diaphragm operation as well
Thanks Bean. I did the vacuum test drive -- I taped the gauge to my windshield -- bet that was a sight) and noted readings (around 14-18 at most cruising speeds/throttle).
I just adjusted the advance can. I'm now at 22 initial, 30 at idle with vacuum hooked up, 34 total (with VA plugged) and 48 all in total at about 2,500 with vacuum advance. I know 22 sounds like a lot of initial but my little 340 loves it (I run a mix of 5 gal 110 and 15 gal 93) and I have no hot starting problems at all. Engine sounds happy. I'm gonna go for a spin and see if I have any stumbles anywhere.
 
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Thanks Bean. I did the vacuum test drive -- I taped the gauge to my windshield -- bet that was a sight) and noted readings (around 14-18 at most cruising speeds/throttle).
I just adjusted the advance can. I'm now at 22 initial, 30 at idle with vacuum hooked up, 34 total (with VA plugged) and 48 all in total at about 2,500 with vacuum advance. I know 22 sounds like a lot of initial but my little 340 loves it (I run a mix of 5 gal 110 and 15 gal 93) and I have no hot starting problems at all. Engine sounds happy. I'm gonna go for a spin and see if I have any stumbles anywhere.
22 isn't a lot by today's standards...it's right where many engines with any mods want to be. Thanks for the report!
 
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