Exhaust port is clean, but how about the exhaust screen? Is the carb getting gas ok? (primer bulb works well?) Or, if there is no primer bulb, check to make sure the gas line is good in the tank. Ethanol will eat gas lines to the point of them falling off, sometimes. Also, sometimes the pickup bulb at the end of the gas line gets gummed up and doesn't draw fuel well. Might need to clean carb. Hope this helps.