The recoil starter assembly on a lawn mower allows the operator to start combustion on the mower's engine. When the recoil starter system is engaged, the operator can crank the engine by hand and get it going fast enough to start all the internal operating systems on the mower. This starter assembly is a critical part to the mower, and without it the mower will not start.
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The Starter Rope
The only visible part of the recoil starter system is the starter rope. This rope sits either on the front handle or on the mower's engine and it allows the operator direct access to the starter system. When the operator pulls out on the starter rope, the rope engages the starting mechanism and gets the engine turning fast enough for the ignition module to generate a spark to ignite the fuel in the cylinder. This starter rope wraps around a pulley system, which allows it to be pulled out and recoil automatically into the engine.
The Pulley System
The pulley system on a lawn mower's recoil starter usually sits below the starter cover on top of the engine. In the center of the pulley, a recoil spring connects to the pulley through a hook. When the starter rope gets pulled off the pulley and out of the engine, the recoil spring stretches out. When the operator lets go of the rope, the recoil spring snaps back and the rope recoils onto the pulley. This allows the operator to continuously pull the rope to get the engine moving faster and faster toward combustion.
The recoil starter system on a lawn mower mainly serves to operate the mower's flywheel. The flywheel sits on the starter side of the mower, on the end of the crankshaft. On the outer ring of the flywheel two magnets help generate magnetic energy as the flywheel spins around the crankshaft. When the magnets build up enough magnetic energy, they connect with the ignition module, firing off the high voltage spark required for combustion. So, as the operator pulls out on the starter rope, they are really turning the flywheel.
The crankshaft is a metal stub sitting in the center of the flywheel that powers all of the mower's moving parts through compression. As the flywheel rotates, it also turns the crankshaft, which helps move the piston up and down inside the cylinder. This up and down pumping pulls more gas and air into the engine, and pushes out heated gases and excess fuel from the system. If the crankshaft can't spin fast enough the entire engine will shut down, so it's crucial to pull hard on the starter rope to get it all moving fast enough for combustion to begin.