I was riding just fine when the engine bogged down and died. I checked the usual carb settings and everything seemed to be fine. When I tried to start it up again it sounded much higher pitch in the exhaust and wouldn't start. I have taken everything apart, cleaned it and checked for leaks. I have spark and gas, there is air flow comming from the exhaust but it still won't start. Seals look good. I am lost as to what to do now. Any suggestions?