Been working on the 40 lately. It has been running great and starting up every time I drive it which is not much.
But have been having this nagging carb backfire ever since I have had the carb rebuilt.
So I started digging around and reading post and everything points to a vacuum leak!
After removing the carb a few times and cleaning the surfaces and installing new gaskets.
Things didn’t get better. I removed the carb again and had walked away from it for a few days.
Got back and checked the studs and two of them were loose. Apparently they had backed out the many times I removed the carb.
Removed them cleaned the surface and re installed them.
And no more vacuum leak!
During all this I had checked and re checked points gap, timing, spark plugs and wires. And they all were functioning fine.
I also found a loose alternator ground wire (white/blue) that was arching and not making good contact why the truck would shut off as I was driving and backfire (tailpipe). Fixed that and it all went away.
Did a lean drop and adjustments and was back up to 19 inches on the gauge.
@mattressking suggested I bump up the timing too 10 degrees to fined more vacuum and sure enough it helped and I am up at 20 inches on the gauge.
So after all that it now runs and idles good with no misfire! But now I have a flat spot or hesitation as I press on the throttle.
I have to feather the pedal to make it go past the hesitation.
Next thing to tackle!