I knew the Chevelle’s Vacuum Advance didn’t actually advance the timing; I figured it was just seized up. I had a new one ready to install, and for fun I figured I’d check if the old one would move the distributor plate at all.

What I found out, however, is that the canister was no only incapable of moving things, but also seemed to have no diaphragm left (as in it would hold zero vacuum no matter how much I pumped my knock-off mighty-vac), meaning it was a substantial vacuum leak.

New one went in without issue and actually A) advances the timing and B) doesn’t leak vacuum, meaning it accomplishes 1oo% of its intended function, 100% more than the original one did.

Car still feels like it has a vacuum leak but it’s better. Still need to re-tune the idle circuit in the carb, which should help more. Idle is pretty high now too, will back that off a touch when I adjust the mixture. Definitely has a lot stronger vacuum.


As a ‘Thank You’ when I started it up after replacing this, the car spit a nice spray of oil out the driver’s side tailpipe. You’re welcome, imaginary valve seals car.