Starts but runs irregularly or misses
1. Spark plugs in bad condition or partially fouled.
2. Spark plug cables in bad condition and leaking.
3. Spark plug gap too close or too wide.
4. Damaged ignition coil.
5. Damaged ignition module or sensor.
6. Battery nearly discharged.
7. Damaged wire or loose connaction at one of battery terminals or coil, or at plug between ignition sensor and module.
8. Intermittent short circuit due to damaged wire insulation.
9. Water or dirt in fuel system and carburetor vent line closed off.
10. Fuel tank cap vent plugged or carburetor vent line closed off.
11. Carburetor controls misadjusted.
12. Weak or broken valve springs.
13. Air leak at intake manifold or air cleaner.
14. Damaged intake or exhaust valve.
15. Incorrect valve timing.
A spark plug fouls repeatedly
1. Incorrect spark plug for the kind of service.
2. Piston rings badly worn or broken.
3. Fuel mixture too rich. See fuel section caburetor troubleshooting.
4. Valve guides badly worn.
5. Valve guide seals badly worn.
Pro-ignition or detonation ( knocks or pings)
1. Excessive carbon deposit on piston head or in combustion chamber.
2. Incorrect spark plug.
3. Damaged spark plugs.
4. Ignition timing advanced.
Valve train noise
1.low pressure caused by oil feed pump not functioning properly or oil screen obstructed.
2. Defective hydraulic tappets.
3. Bent push rod.
4. Rocker arm binding on shaft.
5. Valve sticking in guide.
Excessive vibration
1. Wheels, sprockets and/or tires damaged.
2. Loose or corroded batteryconnections.
3. Ignition module not properly grounded.
4. Primary chain badlyworn or links tight as a result of insufficient lubrication or rear belt/sprockets worn.
5. Upper mounting bracket loose, broken or improperly spaced.
6. Lower mounting bolts loose.
7. Transmission and/ortransmission sub -mounting plate loose in chassis.
8. Broken frame.
9. Internalengine problem.