Motorcycle Repair: starting fault, starter clutch, motor brushes

Question
my bike isn't starting anymore.  The battery, starter, relay/solenoid, rectifier, and fuses have all been changed. everything lights up, i can push start the bike and it runs perfectly.  all it does is click when the starter button is pushed. every now and then, it gives me a weak, breif turn from the starter or motor.  what can be the fault?  

Answer
Hi Jeremy,

The usual cause of this problem are:

Loose battery cable.
Weak or defective battery.
Worn starter motor brushes.

Here's additional possible causes:

Respectfully,
Mark Shively




Engine Doesn't Start, Starting Difficulty

Starter motor not rotating:

Starter lockout or neutral switch trouble
Starter motor trouble
Battery voltage low
Relays not contacting or operating
Starter button not contacting
Wiring open or shorted
Ignition switch trouble
Engine stop switch trouble
Fuse blown

Starter motor rotating but engine doesn't turn over:

Starter clutch trouble

Engine won't turn over:

Valve seizure
Cylinder, piston seizure
Connecting rod small end seizure
Connecting rod big end seizure
Transmission gear or bearing seizure
Camshaft seizure
Balancer bearing seizure

No fuel flow:

Fuel tap vacuum hose clogged
Fuel tank air vent obstructed
Fuel tap clogged
Fuel line clogged
Float valve clogged

Engine flooded:

Fuel level in carburetor float bowl too high
Float valve worn or stuck open
Starting technique faulty
(When flooded, crank the engine with the throttle fully open to allow more air to reach the engine)

No spark; spark weak:

Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap not in good contact
Spark plug incorrect
IC ignitor trouble
Neutral, starter lockout, or side stand switch trouble
Pickup coil trouble
Ignition coil trouble
Ignition or engine stop switch shorted
Wiring shorted or open
Fuse blown

Compression low:

Spark plug loose
Cylinder head not sufficiently tightened down
Cylinder, piston worn
Piston ring bad (worn, weak, broken, or sticking)
Piston ring/land clearance excessive
Cylinder head gasket damaged
Cylinder head warped
Valve spring broken or weak
Valve not seating properly (valve bent, worn, or carbon accumulation on the seating surface)
Hydraulic lash adjuster damaged (worn, seizure, or spring broken)
Hydraulic lash adjuster oil passage clogged