The four-stroke motor is the heart of most modern motorcycles. Although four-stroke motors are available in different displacements and cylinder arrangements, their basic components remain the same.
Also referred to as the top end, the cylinder head houses the pistons, valves, rocker arms and camshafts.
A pair of valves, used for controlling fuel intake and exhaust, are controlled by a set of fingers on the camshaft called lobes. As the intake valve opens, a mixture of fuel and air from the carburetor is pulled into the cylinder. The exhaust valve expels the spent air/fuel mixture after combustion.
Usually chain or gear-driven, the camshaft spins, using its lobes to actuate the rocker arms. These open the intake and exhaust valves at preset intervals.
The piston travels up and down within the cylinder and compresses the air/fuel mixture to be ignited by a spark plug. The combustive force propels the piston downward. The piston is attached to a connecting rod by a wrist pin.
The crankshaft is made up of a left and right flywheel connected to the piston's connecting rod by a crank pin, which rotates to create the piston's up-and-down motion. The cam chain sprocket is mounted on the crankshaft, which controls the chain that drives the camshaft.
The transmission is made up of a set of gears that convert the rotational speed of the crankshaft into torque. The transmission is engaged by the clutch and causes the engine sprocket to spin at the converted ratio, pulling the drive chain and ultimately causing the rear wheel to spin as the chain pulls the rear sprocket.
- The Professional Motorcycle Repair Program, Lesson 1; Professional Career Development Institue, 1995
- The Complete Idiot's Guide to Motorcycles; Holmstrom, Darwin; Alpha Books; 2001
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