How to check vac advance?

[BlackedOut67shorty]

Well im back and i think this could be my issue now, You might remeber i had a really bad oil leak before and carb issues. well it was the pvc hose at an extreme bend that blew out my valve cover gaskets and cause my carb to make it seem like it was running out of fuel. im going through solving these issues and while i was pulling hoses off the vacuum advance, nothing changed. Tried sucking air out of it but nothing changed. Is it bad or am i not checking it right, just replaced it before but i dont know how i can check it without pulling the whole assemblyu out

[camrycurt]

If you can find a hand vacuum pump, hook that up and pull a vacuum on the advance can and see if it will hold the vacuum. I'm not sure if you'd be able to check that by sucking the air yourself - they can be pretty stiff.

Of course, if you can suck air through it easily then it clearly has a leak.

You might also want to check that the hose you're hooking up to it is actually hooked to a good vaccum source on the carb. I used manifold vacuum on mine and it works much better than a ported vacuum source.

[yuccales]

Open the dwell window on your cap, start your engine, look inside the window while you move the throttle and you should see the points plate move. At idle you should be able to pull off the vac advance hose and have no change in idle. If you have a vac tester, hook it up to the advance and test it for leaks. No need to remove it to test it. Or you can suck on the advance hose, and watch the points plate for movement.

[BlackedOut67shorty]

Oh i have an HEI, sorry for not stating that

[truckster]

That still works, except for the having the truck running part. Take the cap off and either use a vacuum pump or suck on the vacuum hose. You should be able to see the movement on the plate in the distributor.

[cleszkie]

FYI - if its an adjustable advance can, the amount of vacuum advance can be adjusted from 0 degrees up to something like 18 degrees. If yours is dialed all the way counter-clockwise (the allen head bolt inside the can) it will be set for 0 degrees advance and all the vacuum in the world will not budge it. So check the setting before you started chasing down other possible causes. Accel has a diagram that shows how much vacuum advance you get from one of their cans for each turn of the allen head bolt out from the fully seated position. Something like 3 degrees of advance for every full turn of the adjuster bolt. Good luck.

[Wrenchbender Ret]

You can also check it with a timing light. When you hook up the hose the timing should advance several degrees. If it is running on ported vacuum you will have to open the throttle a ways.