A motorcycle, often called a motorbike, bike, cycle, or trike, is a two- or three-wheeled motor vehicle. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising, sport, and off road riding. Motorcycling is riding a motorcycle and being involved in other related social activity such as joining a motorcycle club and attending motorcycle rallies.

While this Motorcycle Troubleshooting Guide is not a complete guide of every possible cause for each problem listed, it will help assist mechanics in troubleshooting some of the most common difficulties they will face.  

Engine Doesn't Start, Starting Difficulty: 
-Starter motor not rotating: 
-Starter lockout or neutral switch trouble 

Starter motor rotating but engine doesn't turn over:  
-Starter motor clutch trouble 
-Engine won't turn over: 

No fuel flow: 
-Fuel tap vacuum hose clogged 
-Fuel tank air vent obstructed 

Engine flooded: 
-Fuel level in carburetor float bowl too high 
-Float valve worn or stuck open 

No spark; spark weak: 
-Battery voltage low 
-Spark plug dirty, broken, or maladjusted 

Compression Low: 
-Spark plug loose 
-Cylinder head not sufficiently tightened down 

Poor Running at Low Speed: 
-Battery voltage low 
-Spark plug dirty, broken, or maladjusted 

Fuel/air mixture incorrect: 
-Pilot screw maladjusted 
-Pilot jet, or air passage clogged 

Compression low: 
-Spark plug loose 
-Cylinder head not sufficiently tightened down 

Other: 
-IC igniter trouble 
-Carburetors not synchronizing 

Poor Running or No Power at High Speed: 
-Firing incorrect: 
-Spark plug dirty, broken, or maladjusted 

Fuel/air mixture incorrect: 
-Starter plunger stuck open 
-Main jet clogged or wrong size 

Compression low: 
-Spark plug loose 
-Cylinder head not.sufficiently tightened down 

Knocking: 
-Carbon built up in combustion chamber 
-Fuel poor quality or incorrect 

Miscellaneous: 
-Throttle valve won't fully open 
-Vacuum piston doesn't slide smoothly 

Overheating: 
-Firing incorrect: 
-Spark plug dirty, broken, or maladjusted 

Fuel/air mixture incorrect: 
-Main jet clogged or wrong size 
-Fuel level in carburetor float bowl too low 

Compression high: 
-Carbon built up in combustion chamber 
-Engine load faulty: 

Lubrication inadequate: 
-Engine oil level too low 
-Engine oil poor quality or incorrect 

Coolant incorrect: 
-Coolant level too low 
-Coolant deteriorated 

Cooling system component incorrect: 
-Radiator clogged 
-Thermostat trouble 

Over Cooling: 
-Cooling system component incorrect: 
-Thermostatic fan switch trouble 

Clutch Operation Faulty: 
-Clutch slipping 
-Friction plate worn or warped 

Clutch not disengaging properly: 
-Clutch plate warped or too rough 
-Clutch spring tension uneven 

Gear Shifting Faulty: 
-Doesn't go into gear; shift pedal doesn't return: 
-Clutch not disengaging 

Jumps out of gear: 
-Shift fork worn 
-Gear groove worn 

Overshifts: 
-Neutral positioning pin spring weak or broken 
-Pawl spring broken 

Abnormal Engine Noise: 
-Knocking: 
-IC igniter trouble 

Overheating Piston slap: 
-Cylinder/piston clearance excessive 
-Cylinder, piston worn 

Valve noise: 
-Valve clearance incorrect 
-Valve spring broken or weak 

Other noise: 
-Connecting rod small end clearance excessive 
-Connecting rod big end clearance excessive 

Abnormal Drive Train Noise: 
-Clutch noise: 
-Weak or damaged damper 

Transmission noise: 
-Bearings worn 
-Transmission gears worn or chipped 

Drive chain noise: 
-Drive chain adjusted improperly 
-Chain worn 

Abnormal Frame Noise: 
-Front fork noise: 
-Oil insufficient or too thin 

Rear shock absorber noise: 
-Shock absorber damaged 
-Disc brake noise: 

Other noise: 
-Bracket, nut, bolt, etc. not properly mounted or tightened 

Oil Pressure Warning Light Goes On: 
-Engine oil pump damaged 
-Engine oil screen clogged 

Exhaust Smokes Excessively: 
-White smoke: 
-Piston oil ring worn 

Black smoke: 
-Air cleaner clogged 
-Main jet too large or fallen off 

Brown smoke: 
-Main jet too small 
-Fuel level in carburetor float bowl too low 

Handling and/or Stability Unsatisfactory: 
-Handlebar hard to turn: 
-Steering stem locknut too tight 

Handlebar shakes or excessively vibrates: 
-Tire worn 
-Swing arm pivot bearing worn 

Handlebar pulls to one side: 
-Frame bent 
-Wheel misalignment 

Shock absorption unsatisfactory: (Too hard) 
-Front fork oil excessive 
-Front fork oil viscosity too high 

Shock absorption unsatisfactory: (Too soft) 
-Front fork oil insufficient and/or leaking 
-Front fork oil viscosity too low 

Brake Doesn't Hold: 
-Air in the brake line 
-Pad or disc worn 

Battery Discharged: 
-Battery faulty (e.g., plates sulphated, shorted through sedimentation, electrolyte level too low) 
-Battery leads making poor contact 

Battery Overcharged: 
-Regulator/Rectifier trouble 

Source: cyclepedia.com