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Old 07-14-2010, 09:56 AM
kyle86 kyle86 is offline
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Originally Posted by cobalt327
At light throttle cruise, the vacuum is as high as it is going to get. There's no way to lower the vacuum, or to set the vacuum advance to not be fully advanced at the maximum amount of vacuum that it will ever see, short of at idle. This is going to be the case, no matter what side the limiter is on.

The difference in where the limiter is (in front or behind like the Crane), has NO effect on how much vacuum it takes to first move the vacuum advance rod on the can I'm testing right now. Intuitively it may seem that it would be otherwise, but for that to be the case, the spring inside the vacuum advance can would have to be a progressively wound spring.

At least in the case of the can I just tested, this is not the case- but the real test would be on the one that you're using. What MY cans do or don't do won't help you either way, other than to see if there's a trend. So before assuming anything- check your can w/a MityVac like I just did. My can's marked AR16, BTW.

On my can, the same amount of vacuum causes the initial movement of the rod. Same amount of vacuum to begin the decrease of advance. This is whether the rod has had 1/2 of its travel taken up by a spacer placed in front or behind the rod. The same ~4 in/HG moves the rod, either way.

If there's a reason to, I have numerous vacuum advance cans out in the shop that I will be happy to test tomorrow to see whether they behave the same or not.

In any event, I believe if limiting the total vacuum advance amount that the can is set to provide doesn't do it- then nothing involving the vacuum advance will do it, period.
Have you found that the preload on the spring doesn't cause it to drop out faster if limited from the back (Crane) verses the front?

Say on the AR16, around 4" the arm starts to move and it's total advance is in around 15". If it's limited to 10 degrees the way I did mine, wouldn't the advance start to move around 4", but hit the limiter around say 8" vacuum instead of the total 15" needed to move the whole arm then entire distance?

Then if it's limited to 10 degrees the crane way from the back side of the arm, you would think that it would have to make it all the way to that 15" before all the advance was in regardless of where it started since the arm has to move all the way to it's factory stop right? I don't have access to a vacuum pump right now so I don't know.

Last edited by kyle86; 07-14-2010 at 10:03 AM.
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