Ways to test the Vacuum Advance? - Team Camaro Tech
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post #1 of 6 (permalink) Old Mar 17th, 08, 06:19 AM Thread Starter
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Ray
 
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Ways to test the Vacuum Advance?

I may have a Vacuum Advance on my HEI distributor that is not fully functioning correctly. Does anyone have any ways I can test it to find out if it good, bad, on its way to bad?
Thanks
Ray
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post #2 of 6 (permalink) Old Mar 17th, 08, 06:25 AM
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Jim
 
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Re: Ways to test the Vacuum Advance?

a MitiVac is best. They are cheap and readily available, and useful for lots of other things.

A timing tape, or degreed balancer is very helpful as well as a timing light.

For the most basic test, just check your timing with the vac can disconnected, then connect it to manifold vac and check timing again. he difference is the amount of vacuum advance @ your idle rpm.

With the mitivac, you can check the amount of advance at any vacuum level, regardless of actual manifold vacuum.





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post #3 of 6 (permalink) Old Mar 17th, 08, 06:45 AM Thread Starter
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Ray
 
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Re: Ways to test the Vacuum Advance?

The reason I am asking is that I can not get a repeatable reading using the timing light on the balancer with timing tab and having the vacuum canister plugged into the carb or unplugged and the carb vacuum port capped off. Sometimes I see no change in timing, the next time I will see 4 degrees then 8 degrees then back to 0.
I will do a search on MitiVac and try that because then I can simulate the amount of vacuum and see if it bleeds off and find out if the canister has a leak or not.
thanks
Ray

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post #4 of 6 (permalink) Old Mar 17th, 08, 06:58 AM
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Dave
 
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Re: Ways to test the Vacuum Advance?

With engine idling, make sure the vacuum line to the dist has full manifold vacuum. If it does not, then move it to a port with full manifold vacuum at idle. Also make sure the rubber line itself is in good shape. If it exhibits any cracking or is loose to the fittings, then replace.

Now, with engine idling - simply watch the timing mark. Connect, then disconnect, then connect again - the vacuum line, while watching timing mark. The mark should jump substantially up and down as you connect/disconnect the vacuum line. As you connect the line, the mark would appear to go to top (or above) timing plate. When you disconnect the line, the timing mark should return to your base setting (normally around 8 degrees).

Here's a very easy test - remove dist cap, connect long vacuum line to dist canister and suck on the other end. You should be able to visually confirm the canister mechanism working back and forth.

I would also remove rotor and check centrifugal advance weights. They often are rusted up and will stick. So, take them off, don't lose the little springs, clean everything up real nice and lightly oil, then re-assemble and try setting your timing again.

-

Dave F. in Rhode Island
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Bought my first big-block Chevy in 1970
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post #5 of 6 (permalink) Old Mar 17th, 08, 07:41 AM
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Re: Ways to test the Vacuum Advance?

Quote:
Originally Posted by Chevy-SS View Post
I would also remove rotor and check centrifugal advance weights. They often are rusted up and will stick. So, take them off, don't lose the little springs, clean everything up real nice and lightly oil, then re-assemble and try setting your timing again.

-
Ditto. also, the vacuum can moves (what used to be called) the breaker plate, so make sure it moves freely and easily as well.

It can get weird , particularly if some of the mechanical advance is in at idle. you pull the hose, which removes the vacuum advance... this drops the idle speed, which then removes some mechanical advance...





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post #6 of 6 (permalink) Old Mar 17th, 08, 07:59 AM
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Jim
 
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Re: Ways to test the Vacuum Advance?

Quote:
Originally Posted by Chevy-SS View Post
I would also remove rotor and check centrifugal advance weights. They often are rusted up and will stick. So, take them off, don't lose the little springs, clean everything up real nice and lightly oil, then re-assemble and try setting your timing again.

-
Ditto. also, the vacuum can moves (what used to be called) the breaker plate, so make sure it moves freely and easily as well.

It can get weird , particularly if some of the mechanical advance is in at idle. you pull the hose, which removes the vacuum advance... this drops the idle speed, which then removes some mechanical advance...





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