Two-Stroke Motorcycle Troubleshooting Guide

NOTE:
This is not a comprehensive list, giving every possible

cause for each problem listed. It is designed as a general guide to assist in troubleshooting some of the

more common problems encountered. Also check out the spark plug troubleshooting guide from the good folks at NGK.

Engine Doesn’t Start; Starting Difficulty:

Engine won’t turn over:
Piston seizure

Crankshaft bearing seizure

Connecting rod small end seizure

Connecting rod big end seizure

Transmission gear or bearing seizure

Kick shaft return spring broken

Kick ratchet gear not engaging

No fuel:
No fuel in tank

Fuel tank vent blocked

Fuel tap turned off

Fuel tap clogged

Fuel line clogged

Float valve clogged or stuck closed

Engine flooded:
Fuel level too high

Float valve worn or stuck open

Starting technique incorrect
(when flooded, kick with the throttle wide open to allow

more air to reach the engine.)

No spark; weak spark:
Spark plug fouled, broken, or misgapped

Spark plug cap or high tension wiring faulty

Spark plug cap not in good contact with spark plug

CDI unit faulty

Ignition coil faulty

Magneto damaged

Loose wire connector

Engine stop switch stuck closed

Air/fuel mixture incorrect:
Idle air screw mis-set

Slow jet or air passage clogged or restricted

Air cleaner clogged, poorly sealed, or missing

Starter jet clogged or restricted

Incorrect fuel level

Incorrect jet sizes

Low compression:
Spark plug loose or damaged threads

Cylinder head nuts not torqued properly

Cylinder, piston or rings worn or broken

Piston ring/land clearance excessive

Cylinder head gasket not sealing

Cylinder head warped

Cylinder base gasket not sealing

Cylinder nuts not torqued properly

Reed valve damaged

Poor Running at Low Speed;

Weak spark:
Spark plug fouled, broken, or misgapped

Spark plug cap or high tension wiring faulty

Spark plug cap not in good contact with spark plug

Spark plug incorrect

CDI unit faulty

Ignition coil faulty

Magneto damaged

Loose wire connector

Ignition timing not set properly

Air/fuel mixture incorrect:
Idle air screw mis-set

Slow jet or air passage clogged
or restricted

Air filter clogged, improperly installed or missing

Starter plunger stuck open

Fuel tank vent blocked

Intake manifold clamp loose

Air cleaner duct loose

Incorrect fuel level

Incorrect jet sizes

Low compression:
Spark plug loose or damaged threads

Cylinder head nuts not torqued properly

Cylinder, piston or rings worn or broken

Piston ring/land clearance excessive

Cylinder head gasket not sealing

Cylinder head warped

Cylinder base gasket not sealing

Cylinder nuts not torqued properly

Reed valve damaged

Other:
Drive chain misadjusted

Drive chain not lubricated

Wheel bearings faulty

Brakes dragging

Tire air pressure too low

Poor Running at High Speed:

Incorrect Spark
Spark plug fouled, damaged, or misgapped

Spark plug cap or high tension wiring damaged

Spark plug cap not in good contact with spark plug

Spark plug incorrect

CDI unit faulty

Ignition coil faulty

Magneto damaged

Loose wire connector

Ignition timing not set properly

Air/fuel mixture incorrect:
Main jet restricted or wrong size

Jet needle worn

Jet needle clip in wrong position

Fuel level incorrect

Air jet or air passage restricted

Air filter dirty, improperly installed, or missing

Starter plunger stuck open

Fuel flow to carburetor insufficient

Water or dirt in fuel

Fuel tank vent blocked

Intake manifold clamp loose

Air cleaner duct loose

Low compression:
Spark plug loose or damaged threads

Cylinder head nuts not torqued properly

Cylinder, piston or rings worn or broken

Piston ring/land clearance excessive

Cylinder head gasket not sealing

Cylinder head warped

Cylinder base gasket not sealing

Reed valve damaged

Cylinder nuts not torqued properly

Engine does not rev up properly:
Starter plunger stuck open

Fuel level incorrect

Main jet clogged, restricted or wring size

Throttle valve does not fully open

Air filter dirty

Muffler or exhaust port restricted with carbon deposits

Water or foreign matter in fuel

Brakes dragging

Drive chain misadjusted

Ignition timing not set properly

Overheating

Transmission oil level too high

Transmission oil

viscosity too high

Crankshaft bearing worn or damaged

Knocking:
Carbon built up in combustion chamber or on piston crown

Pre-mix not correct

Low octane fuel

Spark plug not correct

CDI unit faulty

Ignition timing not set properly

Overheating:

Ignition problem:
Spark plug fouled, broken, or misgapped.

Spark plug

incorrect

CDI unit trouble

Ignition timing not set properly

Air/fuel mixture incorrect:
Main jet clogged, restricted or wrong size

Fuel level in carburetor too low

Intake manifold clamp loose

Air filter not installed properly or missing

Air filter duct poorly sealed

Compression high:
Carbon built up in combustion chamber

Carbon built up on piston crown
Aftermarket high compression piston

Excessive engine load:
Brakes dragging

Clutch slipping

Transmission oil level too high

Transmission oil viscosity too high

Drive chain misadjusted

Tire air pressure too low

Inadequate lubrication:
Pre-mix incorrect

Transmission oil level too low

Transmission oil poor quality or incorrect type

Incorrect coolant:
Coolant level too low

Coolant old

Coolant not mixed properly

Cooling component problems:
Radiator clogged

Radiator fins damaged

Radiator cap faulty

Water pump not working

Coolant passageways restricted

Clutch Not Working Properly:

Clutch sipping:
Inadequate clutch lever free play

Clutch cable misadjusted or not working smoothly

Friction plates worn

Steel

plates warped

Clutch springs broken or worn

Clutch

release mechanism sticking

Clutch hub or housing

worn

Clutch not disengaging properly:
Excessive clutch lever free play

Clutch cable misadjusted or not working smoothly

Clutch steel plates warped

Clutch spring

tension uneven

Transmission oil deteriorated

Transmission

oil viscosity too high

Transmission oil level too high

Clutch housing bearing seized

Clutch release

mechanism faulty

Gear Shifting Problems:

Doesn’t go into gear; shift lever doesn’t return:
Clutch

not disengaging

Bent shift shaft

Shift fork bent or seized

Gear seized on

the shaft

Gear positioning lever binding

Shift return

spring weak or broken

Shift return spring pin loose

Shift

mechanism arm spring broken

Shift mechanism arm broken

Shift drum broken

Jumps out of gear:
Shift fork worn or bent

Gear groove worn

Gear dogs and/or dog holes worn

Shift drum groove worn

Gear positioning lever spring weak or broken

Shift fork guide pin worn

Drive shaft, output shaft, and/or gear splines worn

Incorrectly positioned transmission snap ring

Overshifts:
Gear positioning lever spring weak or broken

Shift mechanism arm spring broken

Abnormal Engine Noise:

Knocking:
CDI unit faulty

Carbon built up in combustion chamber or on piston crown

Poor quality fuel or incorrect pre-mix

Spark plug incorrect

Ignition timing not set properly

Overheating Piston slap:
Piston to cylinder clearance excessive

Cylinder or piston worn

Connecting rod bent

Piston pin or piston pin holes worn

Other noise:
Connecting rod small end clearance excessive

Connecting rod big end clearance excessive

Piston ring worn, broken or stuck

Piston seizure or other damage

Cylinder head gasket leaking

Exhaust pipe leaking at cylinder head connection

Crankshaft runout excessive

Engine mounts loose

Crankshaft bearings worn

Primary gear worn or chipped

Abnormal Drive Train Noise:

Clutch noise:
Clutch housing to friction plate clearance excessive

Clutch housing gear to primary gear backlash excessive

Metal chip jammed in clutch housing gear teeth

Transmission noise:
Transmission bearing worn or damaged

Transmission gear worn or chipped

Metal chip jammed in gear teeth

Transmission oil insufficient or incorrect

Kick ratchet gear not properly disengaging from kick gear

Drive chain noise:
Chain adjusted improperly

Rear wheel misaligned

Chain not lubricated

Chain worn

Worn sprockets

Missing teeth from sprockets

Abnormal Frame Noise:

Rear shock absorber noise:
Shock absorber damaged

Loose fasteners

Disc brake noise:
Pads installed incorrectly

Pad surface glazed

Disc warped

Caliper faulty

Master cylinder damaged

Loose fasteners

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

Exhaust Smoke:

Excessive white smoke:
Throttle cable misadjusted

Brownish smoke:
Air filter dirty

Main jet too large or fallen out

Starter plunger stuck open

Fuel level too high