(a) Front drive chain too loose. Refer to paragraph [36].
(b) Engine sprocket loose on shaft.
(c) Excessively loose valve tappets. Adjust (par. [43]).
(d) Engine mounting bolts loose. Tighten.
(e) Transmission mounting stud nuts loose. Tighten (par. [60]).
(f) Spark timing incorrect. Adjust.
(g) Internal parts of engine worn or broken. Notify organization mechanic.
b. Tests to Determine Mechanical Condition of Engine.
(1) Rings and Valves. This is a simple compression test. If possible, make this test with the engine warm. Ignition switch must be off. Crank engine slowly, placing entire weight of body on starter crank. Engine compression should sustain the weight of an average size rider several seconds before crank passes through complete range of its travel. If engine offers little resistance to starter crank in testing either or both cylinders, it is an indication that compression is not adequate in one or both cylinders. No clearance or insufficient clearance at valve tappets; valves sticking in guides; valves seating poorly; cylinder heads leaking; spark plugs loose in heads; piston rings badly worn and/or broken; cylinder and pistons badly worn, or lack of lubrication could cause the above condition. Make external checks first: See that oil is in tank, that spark plugs are tight, and examine around cylinder heads for signs of leaking oil deposits. In extremely cold weather, resistance to the starter crank is increased by “stiff” oil in engine and transmission. Therefore, do not mistake this condition to indicate actual engine compression.
(2) Abnormal Engine Noise. Owing to constructional design of the motorcycle, correct adjustment of the several units and components is necessary for smooth and normally quiet engine operation. Many motorcycles have been “deadlined” and engines exchanged because simple tune‐up service and correct adjustment of components have been neglected. Rough, jerky, and noisy engine action at low speed riding is usually caused by excessively loose front and rear drive chains, or by a transmission being loose in its frame mounting. Fast idling of engine with the front drive chain too loose, may give the false impression that the engine bearings and pistons need replacement service. Engine sprocket loose on its shaft will cause a pounding noise similar to that of badly worn engine bearings. If front drive chain is too tight, or engine sprocket and chain are badly worn, a grinding noise results, seeming to come from the engine. Excessively loose valve tappets will cause abnormal metallic noise in the valve timing gear train and engine crankcase. Spark advanced too far will cause rough engine performance at low speeds as well as spark knock, pounding, and overheating. Engine pounding and rough operation with consequent noise, result from loose mounting bolts. Generator mounted to give insufficient gear‐tooth clearance will cause “howl” in gear case.