Definitely the exhaust leaks causing the prob. The engine actually pulls in some of the exhaust, so it if pulls in fresh air itll make it run lean and backfire.
That seal does come out and go in without splitting the cases. I would recommend throwing the new seal in the freezer for a few minutes before you install it that way itll shrink a little bit. and be easier to install. Use a little white lithium grease around the edges if you want as well.
I have pro circuits on mine and love them, good sound, nice power, but i also got a killer deal of 180 shipped with silencers.... Most of the people on here will tell you toomeys. I'd say get whichever you can get a better deal on.
Is it getting spark?
Do the plugs smell like gas?
Did you turn your fuel petcock off and forget to turn it back on? (It happens, not an insult)
Choke crossover tube come loose?
Hows the compression?
Does it sputter at all or do you get nothing? (Flywheel key)
Kill switch bad? Key on if you still have it? Tether?
oh yeah, check your reeds too.