New to the forum, have backfire issue

[Ike50701]

Hello all, my problem is as follows, with a rundown of what Iv done thus far. My bike is a 2004 GZ250 with a little over 6k miles. When I went to start it this spring, it sprayed raw fuel out of the head gasket. I replaced the gasket torqued everything down per the manual, and it ran great. Then a wiring issue popped up, bad terminal on coil. Fixed that. Ran great for a while. Carb started acting funny, took it apart and cleaned it (was like new anyways) put it back on. Now the reason I did that is because the bike started backfiring on me. Now the backfire is even worse, to the point that the bike kills and will not restart for a little while. Last night I changed the oil bc like a dumbass I left the Petcock on prime and it dumped a cpl quarts of gas into the crankcase. I also put a new plug in it while I was @ it. Just got back from a test ride, roughly 3 miles, ran great till I got back home, backfired and killed as I pulled into the driveway. I'm at a loss, any ideas? I'm a car mechanic, bikes are a different animal to me.