Poor Running at Low Speed:
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
IC 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 bowl too high or too low
Fuel tank air vent obstructed
Carburetor holder loose
Air cleaner duct loose
Compression low:
Spark plug loose
Cylinder head not sufficiently tightened down
No valve clearance
Cylinder, piston worn
Piston ring bad (worn, weak, broken, or sticking)
Piston ring/land clearance excessive
Cylinder head warped
Cylinder head gasket damaged
Valve spring broken or weak
Valve not seating properly (valve bent, worn, or carbon accumulation on the seating surface)
Other:
IC igniter trouble
Carburetors not synchronizing
Vacuum piston doesn’t slide smoothly
Engine oil viscosity too high
Drive train trouble
Brake dragging
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