vacuum troubleshooting/backfiring

[Alk-3]

hey folks. I've got a backfiring issue with my 67 11A when i take my foot off the gas after revving it.
I 'm trying to eliminate the distributer as a possible cause, but i don't know how to tell if the vacuum advance is working or not. I cannot turn the distributer with my hand at all, and when I suck on the vacuum hose I get some air coming through, making me think the vacuum diaphragm is broken. should the distributer turn when vacuum is applied to it? Mine wont turn at all, so I think that's the problem.
The distributer is a lucas 25d
Thanks for any help!

[scott]

the distributor doesn't move, the deck that the points and condensor are mounted to does. so with the cap off, sucking on the vacuum tube should move the deck. if it doesn't remove the deck and check the linkage between the diaphram and the deck.

[Alk-3]

hmmm. well, I took the cap off, and nothing moves when I apply vacuum.
I got the truck running really nice now though. a loose exhaust connection was the culprit. I checked the exhaust over last week, so I figured it was not that, but in the end it was. Lesson learned.
I'm still not sure about the vacuum advance though. I unhooked the vacuum line from the distributer and there is no change in engine performance at all.

[scott]

that dizzy thing is odd. i wonder if a spinning distributor shaft makes it easier for a vacuum advance to advance? i know doing a static check on mine i can get the deck to rotate a couple of degree by sucking on the vacuum line. maybe a real dizzy expert will chime in here.