Having problem that motor will not turn more than 3000rpm's under load. Also when running on the muffs at 2500rpm's every now and then the motor will sputer and drop down to 2000 rpm's for a moment and them return to 2500rpm's. Motor has not been run much in the past two years. Have replaced spark plugs, cleaned fuel filter, checked the spark plug wires and everthing seems to fine. The engine starts and runs at low rpm's very well. Any ideas as to what might be causing the engine to run this way?