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Recoil start (also called manual start, pull start, or zip start) is 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, hopefully successfully firing up before the end of the pull stroke; after the end of the pull is reached, the spring operated reel retracts the rope, either to store it, or to prepare it for another attempt to start. It can take a number of tries to get a poorly tuned or little-used engine to fire properly. If the grip is not returned gently, it can damage the starter by the rope getting tangled in the mechanism. The mechanism works as follows: the rope is not directly connected to the crankshaft itself, but to a reel that is connected the crank by a ratcheting mechanism (specifically, a freewheel clutch, which allows the reel to engage the crankshaft, but not the other way around, similar to a bicycle hub, which allows the pedals and drive sprocket to drive the axle, but the axle can't drive the sprocket and pedals, allowing the operator to rest their legs while the bicycle keeps moving). When the rope is pulled, the ratchet engages the crankshaft and forces it to spin as well. Once the engine starts, the ratchet mechanism allows the crankshaft to keep spinning, while a coil spring inside the reel mechanism which has stored up energy when the rope is pulled out reverses its direction and recoils the rope (hence "re-coil starter"). If the reel were directly connected to the crank and lacked the freewheel mechanism, 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 (this is technically not a recoil starter, since it doesn't re-coil the rope; both are actually forms of pull starter).
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.