so i went to go look at this raptor 350 to see if i wanted to trade my blasty for it and we got there and looked it over and started it up and it started to spit and sputter and back fire. the guy said it would judt need a carb cleaned and he says he knows so much about quads. my dad thinks its the valves and timing cause if it was back firing that would mean the exhaust valve is open when it shouldnt be and the timing is what controls that. what do you guys think would be the problem?