1. Late valve closing. 2. Overload. 3. Incorrect ignition timing. 4. Uneven distribution of fuel among the cylinders. 5. The air filter is clogged. 6. Wear of piston rings.
1. Adjust or lap the valve. 2. Adjust the load. 3. Adjust. 4. Check the fuel injection on the pump adjustment bench and adjust. 5. Blow and clean thoroughly. 6. Replace them with new ones.