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Recoil start (also called manual start, pull start, or zip start) refers to a method of starting an internal combustion engine, usually on small machines, such as lawn mowers, chainsaws, ultralight aircraft and portable engine-generators. Recoil start is also used on some small vehicles such as small go-karts, minibikes, and small ATVs.
The starter consists of a rope with a grip at the end, coiled around an end of the crankshaft. When the rope's grip is pulled, the rope uncoils around the end of the crankshaft, spinning it to crank the engine. After the rope is pulled, the flywheel keeps spinning and the engine starts. If the grip is not returned gently, it can damage the starter by the rope getting tangled in the mechanism. The rope is not directly connected to the crankshaft itself, but to reel that is connected the crank by a racheting mechanism. When the rope is pulled, the rachet engages the crankshaft and forces it to spin as well. Once the engine starts, the rachet mechanism allows the crankshaft to keep spinning, while a coil spring inside the reel mechanism reverses its direction and recoils the rope (hence "recoil starter"). If the reel were directly connected to the crank, once the rope reached its end, the spinning crankshaft would keep turning the reel, which would violently recoil the rope in the opposite direction until it reached its end, where it would likely jam and stall the engine.
There is an older, simpler version of pull starter that has a reel directly connected to the crankshaft, and a rope that is not connected to the reel. The rope is wound around the reel and pulled, starting the engine, but once the end of the pull is reached, it comes free, leaving a loose rope in the person's hand. If the engine fails to start on the first pull, the operator has to re-wind it by hand. This is also done when the engine is shut down to prepare it for the next start.
Easy start feature
Most modern engines have an easy start feature where one of the engine valves is held open by a special cam while the rope is being pulled, avoiding the need to overcome compression. This cam is deactivated at the end of the "pull" so that the engine can fire and run by itself.