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It's just a dust cover to protect the points, but it should never get hot enough to melt like that.
are all the venting hoses hooked up correctly?
Ozone is inflammable.This. The stock evacuation system keeps ozone gas from building up in the distributor. Ozone is highly flammable.
I was a little surprised to see what appears to be rust inside the distributor
I search on “control valve”, but don’t see anything relevant. Is there a better or more specific search term ?Clearly a 78' electronic dizzy. Mine had the cover too, but it was cracked beyond use, I discarded it. If your vacuum hoses/valves aren't operating properly, you could get "farts" through the valve and into the dizzy. Your likely got some fuel vapor in it, it cooled and condensed, and then you farted, I mean your truck farted and it lit and melted. Check operation of the control valve that is supposed to prevent this. There are a few threads on here about this.
No. Ozone is not flammable, if its not flammable then it cannot deflagrate, if it can not deflagrate then its definitely not going be capable of achieving detonation velocities. Some simple chemistry will go a long way in unraveling these mystery's. There IS a vent system on the 78' dizzy but its purpose is to provide a DRY distributor, not one that's ozone free. If the valve fails, fuel vapors can get inside dizzy cap.Ozone, beside the possibility of detonating, is a very powerful oxidant. Again, that is why your distributor came from the factory with a vacuum evacuation system to suck ozone out of the distributor.
That airflow looks like was functioning, the nipple on the small vacuum line is the one melted, implying the air exiting the dizzy after it was heated. I Still need to figure out why the heat in the first place & fix itThe simple description of the venting for the distributor goes like this:
1) One of the hoses in the dizzy cap comes from a little (1/4" ?) stubby pipe that sticks through the passenger side firewall. If you look at the passenger side firewall you'll see a little metal box that acts as a filter and has the stubby pipe going through the firewall.
2) Another hose from the distributor cap goes to a vacuum port on the side your air cleaner. That creates a vacuum inside the distributor cap which, in turn, draws fresh dry air from inside the cab of the truck through the first hose. This system helps to reduce moisture and fumes from screwing with your points (and/or the hall-effect sensor) and the mechanical and vacuum advance parts therein.