Briggs & Stratton motors power many lawnmowers and other low-horsepower devices. Most are single-cylinder, four-stroke engines, meaning that the piston travels up and down inside the cylinder twice to rotate the crankshaft once. The spark plug ignites a mixture of gasoline and air in the cylinder. The burning mixture expands, pushing the piston and its connecting rod down, turning the crankshaft. The intake valve opens and closes to let the air-fuel mixture from the carburettor in, and the exhaust valve opens and closes to let out the burnt exhaust gases. The piston, valves and spark plug each must be in position at exactly the right moment for the motor to run.
Remove the blower covering (cowl) and locate the flywheel. The flywheel is a heavy disc that regulates the speed and consistency of motion of the motor. Check the flywheel for damage such as broken edges or a broken flywheel key. The flywheel key is a rectangular or D-shaped piece of soft metal that locks the flywheel onto the crankshaft. It is soft so it can break away in case of abrupt stopping, thus protecting the crankshaft from damage. Without the key to hold the shaft in the right place, the timing will be wrong, and the motor will run incorrectly or not at all.
Gently try to move the flywheel clockwise and counterclockwise without turning the crank. If it moves even a few thousandths of an inch, replace the flywheel key. The flywheel has a slot in the centre opening that the crankshaft goes through and there is a matching slot in the shaft. The flywheel key locks the two slots together, lining them up exactly.
Remove the flywheel with the flywheel puller designed specifically for your particular machine. This reduces the chance of damage to the flywheel or crankshaft. Most flywheels have two or three holes around the centre of the flywheel. Thread the special puller blocks into the holes and then tighten the nuts until the flywheel pops off of the crankshaft.
Pry out the sheared flywheel key with a large slot screwdriver. Tap the key gently, if necessary, to loosen it. Be careful not the mar the crankshaft. Clean off all areas around the flywheel location carefully
Slide the flywheel back onto the shaft, aligning the slot in the flywheel exactly with the one on the shaft. Tap the new flywheel key into the slot, taking care not to deform the soft metal of the key. Reassemble the rest of the mechanism. The motor is now properly timed and should run smoothly.
Slide the flywheel back onto the shaft, aligning the slot in the flywheel exactly with the one on the shaft. Tap the new flywheel key into the slot, taking care not to deform the soft metal of the key. Reassemble the rest of the mechanism. The motor is now properly timed now and should run smoothly.
Remove the spark plug wire while working on the machine to ensure that the engine cannot accidentally be started. Secure the blade with a block of wood.