ran my sons new motor 2 days ago for 15-20 minutes runs on methanol and then after i ran premium fuel threw it afterwards like i always did on my other animal motor but today when we started the new motor for him to drive the kart around on the street there was a big backfire on the initial start up but then ran just fine. is this common? my old motor never did that so got me a little nervous that the new one did it. new to karting and don't know anything about motors. thanks for any advice.