[C. Clark]

Hello,
Popping, back firing, in my case was carb problems, to lean / to rich, generally out of adjustment or sucking air from the butterfly stems or somewhere. I replaced the Solex carbs with Webers. While I was at it I read here that the .050 dist. was the way to go. Roland Kurtz the resident Porsche guru called the .009 a engine killer, I'm really not sure about that.
Since your tach is acting wierd the dist. may have something to do with it since the dist. produces the signal that is sent to the tach.
The longer I'm around this stuff the more I realize it's either spark or gas in most cases. Those old Solex's can be a real problem.
Good luck and Happy motoring,
Chas.