If you have a lawnmower that is backfiring or pulling the pull start out of your hand, the flywheel key may be sheared. The key in a small engine mates the flywheel to the crankshaft in a certain spot. The flywheel can only fit one way or the timing will be incorrect. The flywheel key can be sheared if a heavy object was hit by the mower, such as a rock. With no key keeping the engine in the right place it will no longer run correctly.
The first step will be to remove any covers, and then the recoil shown in this picture.
Once the recoil is removed you will need to remove the nut that holds the recoil hub on. This will be in the center of the flywheel. You can remove the nut by carefully holding the flywheel with a tool. This may be difficult to do without damaging the flywheel, especially if it is aluminum. The best way to remove the flywheel nut is with a air rachet, then you will not have to hold the flywheel. If the flywheel is damaged the engine will probably not run correctly so be careful.
Your next step will be to decide how to remove the flywheel. The best way is to obtain the correct flywheel puller or a common steering wheel puller. Hopefully the three holes close to the center will be threaded. If you have a steel (cast) flywheel this will be your only removal option. If the holes are not threaded you will need to use a tap on them and thread them. As you can see in this photo the holes are not threaded. This is an aluminum flywheel and here is a slightly risky alternative. You will need these tools.
The trick here is to pry straight across from the flywheel in two spots with prybars. Choose a spot that feels and looks thick and check that nothing under it can be easily damaged. Then take a torch and heat on the flywheel around the crankshaft threads but not on them! When it is hot, apply pressure on the pry bars and softly tap on the crankshaft it should pop loose. Don’t hit the crankshaft hard or you will damage the threads. Also don’t pry too hard or you may break the flywheel. When you have the flywheel loose remove any key that is left in the flywheels keyway.
You can see in the lower two pictures the slot on the crankshaft.
And the damaged key.
Now all you need is a replacement key. Set the key in the crankshaft slot and slide the flywheel back over. Now find out the torque spec and tighten the flywheel nut back over the recoil hub and reassemble the mower. It should start and run smoothly again like it did when it was new.