FLAT SPOTS

If the engine seems to die momentarily when the throttle is opened and then recovers, check for a dirty main jet in the carburetor, water in the fuel, or an excessively lean fuel-air mixture.

POWER LOSS

Poor condition of rings, pistons or cylinders will cause a lack of power and speed.  Ignition timing should be checked.  Bad crankshaft seals may be at fault also.  Sometimes a slipping clutch will give the effect of a power loss.

OVERHEATING

If the engine seems to run too hot all the time, be sure you are not idling it for long periods.  Air-cooled engines are not designed to operate at a standstill for any length of time, as they depend on the flow of air around the cylinder for cooling.  Heavy stop and go traffic is hard on a motorcycle engine.  Spark plugs of the wrong heat range can burn pistons.
An excessively lean gas-air mixture will cause overheating.  Check ignition timing.  Don't let the engine lug by riding in too high a gear.  Broken or badly worn piston rings may permit compres­s.ion gases to leak past them, heating the head and cylinder abnormally.  A low level or the im­proper grade oil in the transmission may cause it to overheat.

BACKFIRING

Check that timing is not advanced too far.  Check fuel for contamination.

ENGINE NOISES

Experience is needed to diagnose accurately in this area.  Engine noises are hard to differentiate between and harder yet to describe.  Deep knocking noises usually mean main bearing failure.  A slapping noise generaly comes from worn pistons.  A light knocking noise during acceleration may be a bad connecting rod bearing.  Pinging, which sounds like marbles being shaken in a tin can, is caused by ignition advance too far or gasoline with too low an octane rating.  Pinging should be corrected immediately or damage to the piston will result.  A compression leak at the head-cylinder joint will make a squealing or puffing noise.

PISTON SEIZURE

Piston seizure is caused by incorrect piston clearances when fitted, fitting rings with im­proper end gap, incorrect fuel-oil mixture (too little oil), incorrect spark plug heat range, or incorrect ignition timing.  Overheating from any cause may result in seizure.

EXCESSIVE VIBRATION

Excessive vibration may be caused by loose motor mounts, loose or worn swinging arm bush­ings, worn engine or transmission bearings, loose wheels, or a broken or cracked frame.  Refer to Chapter Seven which shows a spot on the frame which should be checked regularly if the machine is raced.  See also Poor Handling, (below).
 

CLUTCH SLIP OR DRAG

Clutch slip may be due to worn clutch plates, improper adjustment, or glazed plates.  A drag­ging clutch could result from damaged or bent pla es, improper adjustment, or uneven clutch spring pressure.

POOR HANDLING

Poor handling may be caused by improper tire pressures, a damaged frame or swinging arm, worn shocks or front forks, the installation of wrong shocks for type of riding and/or rider weight, weak fork springs, a bent or broken steer­ing stem, misaligned wheels, loose or missing spokes, worn tires, bent handlebars, worn wheel bearings or axle spacers, or dragging brakes.