Christopher Reichle suggested a burnt exhaust valve. I don't have a
compression gauge (maybe now is a good time to go get one...), but I set
the valve lash to spec at the same time as I was fiddling with the
dizzy. Two or three valves were tight, but I figured that was just
normal valve seat recession. Anyway, I figured a burnt exhaust valve
would be an obvious and sudden condition. Am I wrong?
Well, what I meant by burnt is general failure in that area. I had a
gradually worsening case of backfire which proved to be 2 cracked exhaust
valve seat. As the crack grows it gets worse and you have more of a leak and
a bigger backfire. Compression and vacuum test will show it.