The Stovebolt Site Forums The Shop Area Engine Shop Vacuum advance

With the help of advice from this forum I used a vacuum gauge and set the idle and timing on my 52 pickup. Now I am trying to troubleshoot the vacuum advance. The engine is a 235 from a BelAir. When I rev the engine the advance does not move the distributor. I can manually move the distributor with no problem. Vacuum at the manifold is about 19 at idle.

I disconnected the steel line that comes from the throttle body and checked the vacuum coming to the advance diaphragm. At low idle vacuum is zero and as I rev up the engine it goes to 10-15. I connected the vacuum advance with a rubber hose to the manifold vacuum and it would move the distributor. I tried sucking on the hose to the advnce and could suck air fairly freely.

Do I need a new advance mechanism?

Am I reading this right that your advance works with a rubber hose and not the steel line? Are you pulling vacuum from the same place with the rubber and steel hose. If you are pulling vacuum from the same place and it works with a rubber hose but not the steel line, your steel line must have a hole in it. Or maybe in am misreading your post and we are not on the same page.

After you suck on the hose going to the diaphragm, put your tongue over the end of it. The vacuum which you have just created should hold your tongue to the hose until the cows come home. Sounds like you have a minor leak in the diaphragm. Applying the increased vacuum from the big port is indicating that the leak in the diaphragm is minor and is overcome by the huge increase in vacuum.
Carl

Diaphragm leak and the problem with rubber hose is, its flexible and would naturally negate holding vacuum pressure. Its why all of them use thin metal tubing from the advance around to the carb.

The vendors have the vac line in regular or stainless or you can have one made. Then its a matter of the diaphragm or a leak somewhere. 19 is plenty to move the advance.

Stand corrected, read Jerry's reply.
dg

Only since I have the pictures, here's what that vacuum advance diaphragm looks like on the inside.

I already shared these pictures, but it's a good visual learning aid.

Brad

Ditto on the hand held vacuum pump. The days are long gone when I use my sucker to test for vacuum/fuel flow/leaks........

I added a vacuum pump/self brake bleeder kit to my arsenal a couple yrs back and have used it extensively since. I can't believe I waited so long to buy one. A good kit ain't expensive either.

Dave

Thanks for all the advice. It definitely sounds like the diaphragm is leaking on my vacuum advance. There were some helpful pictures in previous posts that showed the difference between the truck and the car mechanism. Mine is definitely the car part. Those pictures saved me from ordering the wrong part. This site has helped me so many times!

Vacuum advance is not an either/or situation with centrifugal. At idle, there is NO vacuum advance at all, because there's no ported vacuum at the carburetor port. At steady throttle cruise, the vacuum advance ADDS about 8-10 degrees MORE advance than the centrifugal advance provides. If the throttle is opened for acceleration, passing, hill climbing, etc., the vacuum on the diaphragm goes away and the only advance is centrifugal. Retarding the spark a few degrees during acceleration prevents detonation from over-advancing under load.

Initial advance, centrifugal advance, and vacuum advance add up to give a total advance number at light-throttle cruise.
Jerry