I have two of these motors and use Amsoil at 100:1 mix all year long and they just keep on humming. If you have modified the choke lever in any way make sure you verified it stays in the open position during flight. Thats what happend to me, kept getting richer and richer but yet when I landed the choke was wide open. Air pressure was closing the choke during flight. If all else fails send it back to Horizon, they have great customer service. Thanks, Jerry.