f-20 firing issue

[dirtyred]

well I got the f-20 running good on 3 cylinders it is timed right mag is reworked fuel tank is cleaned carb is clean BUT is still misses then back fires you can ground out the #4 plug and it never misses a beat hook it back up and back to running like crap looks like it is firing every other time and I believe that it is double gassing then firing causing the backfire got new plugs new cap only thing left is a new wire but they are copper wire that have been soldered any suggestions

[Gary Dotson]

Have you gotten anywhere on this problem? I was going to post a few days ago, but got quite busy. I can think of several items to check, including checking for a valve that might be sticking or a vac. leak, but the first thing I'd try is to swap spark plugs between #4 and one of the other cyls. to see if the miss moves with the plug. You could have gotten a bad plug.

[dirtyred]

I will swap plugs then get back with yall been working on a cub