Not having ever seen what it is doing it's hard for me to say much about
your distributor. But your timing should advance to a maximum of about 37° and
remain fairly steady at that point. Random flashes usually indicate spurious
tracks in the distributor cap, sparks from one wire to another or other interference.
If the pulley is shiny and light-colored you can see the mark of a sharpie.
I also use chalk. Doesn't last long though....
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George Downs, Bartlesville, Heart of the USA!