Within the past 2 months podcasts. It goes like this: Car is configured w/distributor and points. The problem is that the cap explodes on start-up. Why? The claimed reason is that the vacuum advance actuator has a leak in its diaphragm, and this leak allows enough fuel to leak inside the distributor to cause the cap to blow off at the next start-up, ignited by a spark at the points presumably. Wouldn’t there have to be something else amiss as well? The carb port to the vacuum advance is usually from the air horn above the throttle plates, and that area wouldn’t normally have much fuel after the engine is turned off. Could happen I suppose , but if it did, I’d guess there was a second culprit, not just a leaky vacuuum advance actuator. What do you think? If you concur there’s a second culprit, what do you think it is?
Good problem for “office talk” becaues we can’t check the diaphragm. A leaky “O” ring seal might allow gases from the crankcase to get up there. And on and on.
The mechanic knows for sure the diaphragm has a leak b/c they tested it with a hand-held vacuum pump. I’ve had to deal with diaphragm leaks on those on my truck, which has two, advance & retard, and the distributor cap has never exploded.
It was “poetic license”
So just something interesting that might happen? I can see that. I’d say the most interesting (and puzzling) puzzler of late podcasts is how long it takes an electron to travel the entire circuit from battery to starter motor and back again to the battery. Very surprising answer.
Doesn’t have to be points that sparks, as long as there is an arc from the rotor button to the distributor cap and there is gas fumes present then an explosion of some magnitude can happen…
I think you have to have a perfect storm of sorts as it seems to be a very rare occurrence, like a carb to blow fumes out the vacuum port (small backfire maybe, or the timing being to far advanced causing the engine kicking back/reverse rotating) at shut off as well as a failing vacuum advance pod allowing said fumes into the dist and then it still be their and not evaporated or whatever the next time you start the vehicle…
Vacuum to the distributor is also often from under the throttle plates.
But the vacuum canister is side mounted on the distributor with no path into the distributor to be sparked by the points that I have ever seen. The diaphragm is vented to the atmosphere outside the cap.
Now maybe fuel in the crankcase oil caused by an over rich carb might be forced up the distributor shaft into the cap due to failing rings and ignite. Seems a stretch.
Let’s put it this way - anyone here ever see an actual exploded distributor cap? Not me.
Me personally, never heard of it from anyone nor seen it myself…
Post number 4 and 6… I lost interest after that… lol
Vac advance control is sourced from above the plates for my truck. Little to no vacuum at idle. Vacuum increases w/higher rpm. Truck also has vac retard, sourced from below plates. Hot Rod magazine said to use intake manifold vacuum to control the vacuum advance for more power.