Engine is Hard to Start or Will Not Start / Low Compression
| Valve clearance too tight | Adjust valves |
| Valves not properly seated | Valve job |
| Valve guides worn | Replace valve guides |
| Piston rings damaged or worn | Replace piston rings |
| Piston damaged or worn | Replace piston |
| Cylinder damaged or worn | Replace or bore cylinder |
| Sparkplug fouled or loose | Replace or tighten |
| Electric starter turns over slow | Starter/starter clutch bad or weak battery |
Engine is Hard to Start or Will Not Start / Vehicle has No Spark
| Sparkplug fouled | Replace sparkplug |
| CDI unit faulty | Replace CDI unit |
| Flywheel faulty | Replace flywheel |
| Ignition coil faulty | Replace ignition coil |
| Pulse coil faulty | Replace pulse coil |
Engine is Hard to Start or Will Not Start / Carburetor is Not Getting Fuel
| Fuel valve clogged | Clean fuel valve |
| Fuel valve screen clogged | Clean fuel valve screen |
| Fuel line pinched or clogged | Inspect fuel line |
| Fuel tank vent hose pinched or clogged | Inspect vent hose |
| Float valve stuck | Clean carburetor |
Engine Starts but Shuts Off
| Sparkplug fouled | Clean sparkplug |
| Faulty flywheel | Replace flywheel |
| Faulty CDI unit | Replace CDI |
| Incorrect valve clearance | Adjust valves |
| Clogged carburetor jets | Clean carburetor |
Engine Idles Poorly
| Sparkplug faulty | Replace sparkplug |
| Valve clearance out of adjustment | Adjust valves |
| Valve seating or guides faulty | Do valve job / rebuild |
| Flywheel faulty | Replace flywheel |
| CDI unit faulty | Replace CDI unit |
| Ignition coil faulty | Replace ignition coil |
| Carburetor jets clogged | Clean carburetor |
| Carburetor float set incorrectly | Adjust carburetor float |
| Mixture screw set incorrectly | Adjust mixture screw |
Engine Runs Poorly at High Speed
| Valve timing incorrect | Reset timing |
| Weak valve springs | Replace valve springs |
| Electronic timing faulty | Replace CDI unit |
| Sparkplug gap incorrect | Adjust sparkplug gap |
| Carburetor float level too low | Adjust float level |
| Air filter contaminated | Service air filter |
| Fuel hose pinched | Replace fuel line |
Excessive Exhaust Smoke
| Worn out piston & rings | Replace piston & rings |
| Cylinder worn | Replace cylinder or bore |
| Valve stem seals faulty | Replace valve stem seals |
| Valve guides worn | Replace valve guides |
Engine Lacks Power
| Sparkplug fouled or incorrect gap | Replace sparkplug |
| Valve clearance out of adjustment | Adjust valves |
| Valve timing out of adjustment | Replace CDI unit |
| Piston, rings, cylinder worn out | Rebuild top end |
| Carburetor float level incorrect | Adjust float level |
| Carburetor jets clogged | Clean carburetor |
| Air filter contaminated | Service air filter |
| Engine oil overfilled or contaminated | Change oil & filter |
| Air leak bad seal or gasket | Pressure test engine |
Excessive Noise Coming from Valve Area
| Excessive valve clearance | Adjust valves |
| No oil to the top end | Add oil |
| Camshaft damaged | Replace camshaft |
| Weak or broken valve spring | Replace valve spring |
Excessive Noise Coming from Piston Area
| Piston slapping | Inspect & rebuild top end |
| Piston pin knock | Inspect & rebuild top end |
Excessive Noise Coming from Cam-chain Area
| Cam chain stretched | Replace cam chain |
| Cam sprockets worn | Replace cam sprockets |
| Cam chain adjuster defective | Replace cam chain tensioner |
Excessive Noise Coming from Clutch Area
| Weak clutch springs | Replace clutch springs |
| Broken drive plates | Replace clutch |
| Clutch hub splines worn | Replace damaged components |
| Broken clutch basket | Replace clutch basket |
| Clutch basket fingers worn | Replace clutch basket |
Excessive Noise Coming from Crankshaft Area
| Connecting rod side to side clearance | Replace thrust washers |
| Lower connecting rod bearing worn out | Replace bearing |
| Crankcase bearing worn out | Replace bearing |
Excessive Noise Coming from Transmission Area
| Gears worn or broken | Rebuild transmission |
| Splines worn | Replace worn components |
| Primary gears worn or broken | Replace gears |
| Bearings or bushings worn out | Replace bearings or bushings |
Transmission Will Not Shift
| Shift shaft is bent | Replace shift shaft |
| Gear shift cam broken | Replace gear shift cam |
| Shift forks bent | Replace shift forks |
| Clutch cable out of adjustment | Adjust clutch cable |
Transmission Will Not Return
| Shift shaft is bent | Replace shift shaft |
| Shift forks bent | Replace shift forks |
| Gearshift return spring broken | Replace gearshift return spring |
| Reverse shift cam broken | Replace shift cam |
| Clutch cable out of adjustment | Adjust clutch cable |
Transmission Skips or Jumps Out of Gear
| Shift forks bent | Replace shift forks |
| Transmission gears damaged | Replace damaged gears |
| Cam stopper spring weak or broken | Replace spring |
| Gearshift stopper worn or broken | Replace gearshift stopper |
Clutch Slips when Taking Off
| Clutch cable out of adjustment | Adjust clutch cable (no free play) |
| Clutch springs weak | Replace clutch springs |
| Clutch drive plates worn or damaged | Replace clutch drive plates |
| Pressure plate worn or damaged | Replace pressure plate |
Clutch Drags when Taking Off
| Clutch cable out of adjustment | Adjust clutch cable (too much free play) |
| Clutch springs weak | Replace clutch springs |
| Pressure disc plates worn or damaged | Replace clutch plates |
| Clutch release mechanism worn or damaged | Replace clutch release mech. |
Engine Overheats
| Engine oil low | Add engine oil |
| Carbon build up | Decarbonize exhaust and piston |
| Low grade gasoline (octane) | Use higher octane fuel |
| Fuel level low in carburetor | Adjust float level |
| Intake manifold leaking air | Repair intake leak |
| Coolant level low | Add coolant |
| Fan motor faulty | Check fuse/replace fan motor |
| Fan switch faulty | Check fan switch/replace fan switch |
| Thermostat stuck open | Replace thermostat |
| Radiator cap or hose’s faulty | Replace radiator cap or hose |