The Stovebolt Site Forums The Shop Area Engine Shop Distributor / Vacuum Advance problem

Was out joy-riding when my '51 pickup backfired and quit. The distributor cap was off on one side and the rotor out of timing position. No tools with me, so was towed home. Loosened the distributor and re-indexed to proper position - truck starts but upon acceleration, blows the cap off. Local mechanic (experienced) says the vacuum advance diaphragm is ruptured - gas fumes get in the cap - point ignition sets off a minor explosion in the cap and blows it off. Now, what is the best fix - whole new assembly or can a cheaper part replacement get me back up to speed?
Curious in WV. Thanks, BW

a new vac advance is maybe $20-30.

This is a new one on me! Is this possible though? The vacuum line should be always pulling the air in, right? If so, how can the fumes get into the distributor?

Perhaps a backfire can cause a negative manifold vacuum?

Me too. But let's see. When you accelerate, vacuum goes "down". If you have excessive crankcase pressure, you can actually get pressure in the line. If there's (and this is a stretch, but I'm reaching here) a lot of gas in your oil and the fumes get in there, could this cause a small pop from the points?

This would be one I would ask someone really good, such as Marlin Davis at Hot Rod magazine. He likes getting these wierd problems to solve.

Lemme build on that. Supposing your rings are really worn and you have a lot of blowby. You'll have a lot of gas in your oil, aside from burning oil, and you'll have a VERY high crankcase pressure. Could be simply a symptom of a tired motor ready for a rebuild.

That sounds logical to me. The other day, I was messin' with my motor and somehow I made it backfire so hard that it made (blew) my vacuum windshield wipers over to the other side of it's sweep.

I don't belive the vacuum advance is exploding the cap. Check the side to side play and end play in the distributor shaft. I think the rotor is hitting the cap when you rev the engine.

End play problems are a good thing to check. Or the spring-clips on the dist body are out of shape, or the cap's clip ears are mangled. Check for weird wear on the rotor button or inside cap.

Ralphy,

Thanks for the reality check.

Tim

Agreed - thanks Ralphy. No way for fumes to go thru the vac advance directly into the distributor. I completely missed that one.

Must be a mechanical issue.

Had an almost perfectly running 235 in my 49 but it would occasionally run a bit off. Tried everything and finally discovered (by accident) that the vaccum tubing flare on one end was a bit rusted and the flare was cracked. I tried to flare the cut off but it was too rusty and I replaced the tube. Runs perfectly now. Maybe coincidence maybe not but worth a try.

Good point Ralphy. I was having a little trouble with that theory myself and not yet being entirely familiar with the 235, etc.

Every time I get a "strange" problem, I post it here to get feedback before I venture too far in my efforts to correct the problem. This time my local, friendly mechanic was wrong about the vacuum advance allowing fumes in the dizzy cap.
I did not think it was possible, but he had not taken a good look at the set up and was speculating. The problem was mechanical - worn out dizzy bumping the cap off. I put in a "junkyard" unit and am back on the road. Thanks for the input and "steering" me in the right direction. This is a good forum! BW