backfiring?

So I bought a PMR stage 2 kit from hotrodminibike.com. I got the old air filter and muffler off the engine and put on the UNI air filter and straight pipe header. Also put a new jet in the carburetor(not sure what size....what ever jet and e-tube came with the kit). I fired the motor up(not...

I have a honda clone (6.5hp) that I am running on my mini-bike. The motor is bone stock, but now when I run it all out and reach the top speed, it starts popping (backfiring) then when I slow down it back fires some more. But otherwise it is good. What is causing this? And how can I fix it...