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Main differences are
4 stroke uses mechanical valves to control airflow into and out of the cylinder, 2 stroke uses ports between the cylinder and crankcase to control airflow.

4 stroke only makes compression only compresses and burns the air/fuel mixture one time every 2 revolutions of the crankshaft, 2 stroke compresses and fires on every revolution.

most all 4 strokes use oil in the crankcase to lubricate the transmission,crankshaft,connecting rod bearings,piston, camshaft valves rocker arms(unless it's a dual overhead cam which rides dirrectly on the valves), and everything else in the engine that needs oil. 2 strokes only use oil in the transmission, then use oil mixed with the fuel to lubricate everything that needs it, as the mixture gets moved at high velocity from the intake, thru the crankcase, then back to the cylinder to be burned.

since a 2 stroke fires the compressed air/fuel twice as often as a 4 stroke at the same rpm, 2 strokes normally make about the same amount of power as a 4 stroke twice it's size. Since there's no camshafts, mechanical valves, oil pumps, timing chains, cam timing gears, or valve covers on a 2 stroke, they usually are much lighter than a 4 stroke engine of the same size.

2 strokes usually respond much better to modifications, and are usually cheaper to modify than a 4 stroke, because there's less parts buying involved.
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