curious about vaccum advance

I never really payed to much attention to the vaccum advance on my 258. Its just always kinda been there, and I assume always working. Just wondering, do these things ever stop functioning, and is there a way to test/fix them?

 

Do you have a timing light? Hook up a timing light and disconnect advance. The timing should not move when you speed the engine up.

edit: nevermind - you still have centrifugal advance weights inside that can advance it some. Skip my .02$

 

There's a rubber/plastic diaphragm in them that can leak. Then they either stop working or don't fully advance. I suppose the moving parts could get gunked up and stop working.

Take the distributor and rotor cap off, apply something like a Mity-Vac to the nipple, and see if the points plate (or reluctor plate) rotates. The plate should stay rotated as long as vacuum is applied.

 

I suppose it could fail. But i would almost guarantee the failure would be in the hose that carries the vacuum. The advance mechanism is basically a spring, a diaphragm, and a rod. As the vacuum increases as you push the gas the diaphragm pulls a rod that pulls the pickup in the distributor and advances your timing. Pretty reliable. To test i would probably remove the vacuum line and plug it at the carb, then go for a test drive, maby time how long it takes to get to 45. Then reinstall the line and try the test again. if no performance is gained I would think it may not be working.

 

I think you have the operation a little backwards. When throttle is applied you reduce the vaccum in the manifold. The net effect is still the same, but just thought I'd point that out.

As an additional question, what is the contribution of the vaccum advance? 1 degree? 2 degrees? Is it equal to the contribution from the mechanical advance, greater, less?

 

Yup that is true about vacuum being reduced in the manifold at high throttle positions, that is also why they use ported vacuum (above the throttle plate) for the vacuum advance