The most common pocket bike engines are gas-powered, though electric-powered pocket bike engines do exist as well. Gas-powered engines are usually two-stroke engines — though four-stroke engines are becoming more common — that range anywhere from 25cc to 100cc, depending on the size of the pocket bike. Electric motors can range anywhere from 100 watts to 1000 watts, and anywhere from 24 to 60 volts, again depending on the size of the pocket bike. Most early-model pocket bikes that came out of Japan as toys or consumer bikes were electric-powered, but the earliest versions of pocket bikes, which were pit bikes at races, were gas-powered.
Pocket bikes came into existence at races for full-sized motorcycles. The tiny bikes were used as pit bikes — small bikes made from spare parts that were useful for getting around in the tight pit areas. Since they were made from spare parts, the original pocket bike engines were gas-powered and often constructed from the same materials as full-sized bikes, only on a smaller scale. The power of the gas-powered pocket bike engines are measured in cc; the smaller the cc, the less powerful the engine is. The smallest pocket bike engines are usually around 25cc, while the largest pocket bike engines measure about 100cc. The larger engines are usually reserved for larger bikes.
Gas-powered engines on pocket bikes are usually two-stroke or four-stroke. Two-stroke engines finish the combustion process in one revolution of the crankshaft--or, two strokes of the piston. Four-stroke engines do the same process in four strokes of the piston or two full revolutions of the crankshaft. Two-stroke engines are more commonly used for pocket bikes, as two-stroke engines are used more for small applications such as chainsaws. Two-stroke engines are cheaper and lighter than four-strokes, but they tend to pollute more than four-strokes and they do not last as long as four-stroke engines. The chamber fires every stroke, which means two-stroke engines tend to have more of a power boost, but they also tend to burn gasoline more quickly.
Early Japanese models of pocket bike engines were affixed to toy-like pocket bikes made for racing and recreation. The motors were magnetic brush models, and while not necessarily as powerful as gas-powered engines, they suit the small size of the bikes and did not require gasoline to run. The motors used were similar to the motors used on scooters or power-assisted bicycles. While still available, they are far less common on pocket bikes than gas-powered engines.