I swaped carbs the other night and when I put the new one on I heard a pinging sound, after a while of letting it run it ran perfect. After about 60miles it started backfiring like crazy through the carb at about half throtle. It started all at once without warning. It idles fine. I replaced the mixture control selenoid seal and tested the selenoid and it worked fine and I also tore the carb apart and cleaned it very throughly. Any suggestions would be appreciated?

 

Check the ignition timing...

 

checked the timing and it was dead on. I'm goin to check all the plugs and wires tomorrow, right now I'm thinking that maby a valve bent?

 

found out what it was. Something went down the intake and into the cylinder it messed up the spark plug but compression and everything else was good so it was a $2.00 fix ya'll can delete this post if you want thanks