1. Engine is too Rich. Locate Carburetor and look for the highspeed adjustment screw on the bottom of the carburetor. Reset it by turning the needle in (clockwise) until finger tight. DO NOT OVER TIGHTEN. Back out the adjustment screw counter clockwise 1 1/2 turns to get the primary adjustment. If the engine runs erracticly (surging), with the engine still running, turn the adjustment screw counter clockwise until the surging stops and before the engine runs erracticly again (rich). If you need more guidance, fill out the question form.
2. Dirty or plugged air filter. Replace or clean air filter.
3. Worn or damaged rings. Rings could be worn past tolerances due to old age and use. I recommend taking the engine in to the shop if this is the case because tearing an engine down a putting it back together can take a while and some experience.
4. Scored cylinder. Caused by ingesting dirt. Either a very dirty air filter or a missing air filter will cause this symptom. Carbon from the cylinder head or piston head can get lodged between the piston and cylinder wall, causing scoring.
5. Wrong grade of oil. Oil that is less than 30w HD will vaporize if used for a long time. Use only use a good grade of 30w HD oil for all 4-cylce lawn equipment.
6. Worn valve guides. Valve guide is wore out due to excessive wear on the engine and must be taken to the shop for a replacement guide bushing. Common causes are mowing on a hill side for excessive amounts of time and debris build up on the cooling fins.
7. Choke is still on. Open the choke as soon as the engine fires and continues to run without faltering.
8. Too much oil. Correct the amount of oil in the crankcase. Fill only to the "Full" mark.
9. Loss of crankcase vacuum. Make sure the dipstick or fill plug is secured tightly. Also check for bad O-rings, gaskets and washers.