Four-Stroke Engine

A four-stroke engine needs four piston strokes – two turns of the crankshaft – for one complete working cycle:

• Intake: the piston moves down and the intake valve lets the fuel–air mixture in.
• Compression: both valves close and the piston moves up, compressing the mixture.
• Power: the spark plug ignites the mixture and the expanding gas pushes the piston down.
• Exhaust: the exhaust valve opens and the rising piston pushes burnt gas out.

Compared with a two-stroke, a four-stroke is more fuel efficient, cleaner and gives a broader spread of torque, but it is heavier and more complex, with valves and camshafts. It is used in almost all road motorcycles and scooters and most modern motocross and enduro bikes. Check oil level and valve clearances as the manual says; four-stroke engines have separate engine oil, not oil mixed in the fuel.