I bought a 400ex little while ago and it start to have problems when I turn the gas on on it runs really bad it has no power it just dies out then I turn it on res. it runs fine and on bolth settings it builds up gas in their air filter box then after riding for a little when I turn it of then turn it back on all it dos is smoke when trying to start it and it dosent start up idk whats wrong with it but please help