Spark weak:

Battery voltage low
Spark plug dirty, broken, or maladjusted
Spark plug cap or high tension wiring trouble
Spark plug cap shorted or not in good contact
Spark plug incorrect
igniter trouble
Pickup coil trouble
Ignition coil trouble

Fuel/air mixture incorrect:

Pilot screw maladjusted
Pilot jet, or air passage clogged
Air bleed pipe, bleed holes clogged
Pilot passage clogged
Air cleaner clogged, poorly sealed or missing
Starter plunger stuck open
Fuel level in carburetor float chamber too high or too low
Fuel tank air vent obstructed
Carburetor holder loose
Surge tank duct loose

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
Engine not sufficiently warmed up after lash adjuster installation
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

Backfiring when decelerating:

Vacuum switch valve broken
Air suction valve trouble
Coasting enricher trouble


Igniter trouble
Carburetors not synchronizing
Carburetor vacuum piston doesn’t slide smoothly
Engine oil viscosity too high
Drive train trouble
Final gear case oil viscosity too high
Brake dragging
Air suction valve trouble
Vacuum switch valve trouble


Amateur radio operator from Malaysia

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