Engine fails to start or is difficult to start—possible cause may be :-

• Throttle opened too far.
• If cold, air lever in open position, or bad air leak at carburetter flange.
• If cold, lack of fuel due to insufficient flooding of carburetter.
• Lack of fuel due to stoppage in pipe, taps, or air lock in tank.
• If engine hot, excessive flooding of carburetter.
• Carburetter pilot system choked.
• Sparking plug H.T. lead come adrift.
• Sparking plug dirty or oiled up.
• Sparking plug gap incorrectly set.
• Sparking plug external insulator dirty or damp.
• Sticking valves.
• Broken valve spring.
• Valves not seating properly.
• Contact points dirty or incorrectly adjusted.
• Dirty or wet high tension brush holder and lead.
• Dirty slip ring or stuck pick-up brush.

Engine misfiring—possible cause may be :-

• Air valve not opened fully.
• Defective or oiled up sparking plug.
• Contact breaker incorrectly adjusted or points loose.
• Tappet clearance incorrect.
• Contact breaker points dirty or oily.
• Valve springs weak.
• Fuel supply restricted due to blockage.
• High tension lead to plug faulty or insecurely fitted to magneto or suppressor.
• Carburetter float punctured.

Loss of power—possible cause may be :-

• Sparking plug faulty or unsuitable type.
• Insufficient oil in tank, or dirty or unsuitable oil.
• Oil pump not primed.
• Valve springs weak or broken.
• Valves sticking or not seating properly.
• Valve or ignition timing incorrect.
• Piston rings worn or broken.
• Engine excessively carboned.
• Brakes sticking on or too closely adjusted.
• Silencer tampered with.

Engine overheating—possible cause may be :-

• Faulty lubrication. Insufficient, dirty, or unsuitable oil.
• Oil pump not primed.
• Sparking plug faulty or unsuitable type.
• Air valve not opened fully.
• Carburetter float punctured.
• Engine excessively carboned.
• Valve springs weak or broken.
• Valves sticking, or not seating properly.
• Piston rings worn or broken.
• Valve or ignition timing incorrect.
• Brakes sticking on or too closely adjusted.

Engine stops suddenly—possible cause may be :-

• Lack of fuel in tank or supply obstructed.
• High tension lead come adrift.
• Carburetter choked.
• Carburetter float chamber air locked due to choked vent.
• Fuel tank air locked due to choked vent in cap.
• Sparking plug oiled up, or points bridged.
• Water on high tension pick-up or sparking plug.
• Water in float chamber.

Excessive oil consumption—possible cause may be :-

• Suction filter in crankcase restricted.
• Fabric oil filter element requires replacing.
• Air leak into crankcase at bottom.
• Ball valve leaking.
• Cylinder, piston, and rings worn.

Excessive fuel consumption —possible cause may be :-

• Fuel taps, pipe, or unions leaking.
• Level too high in carburetter.
• Punctured float.
• Incorrect use of air lever.
• Needle jet worn.
• Needle incorrectly set.
• Pilot air adjustment too rich.
• Valve or ignition timing incorrect.
• Brakes sticking on or too closely adjusted.