Problem is that it backfires through the exhaust not carb side.

Originally the problem was a blown head gasket since during a ride w my dad my dtre over heated and blew the gasket, i got new gaskets and also resurfaced the head, yet still has the same peoblem

I've confirmed it gets gas, spark and everything else it needs.

Ive tried
New plug
New coil
Adjusting the timing (i have HPI igniton)
New reeds
Also has a new piston set.

I need to fiqure out whats the problem so i can finally start going to work with it.

Any suggestion or advice is really usefull