Showed the two flywheels to a mower mechanic today. He agrees with me that it hasn't hit anything. One fin has failed and then it was like a domino effect, taking out the next three and half. There is a much longer gap between the fins on the more modern flywheel, so the spring really has a chance to unwind its energy, so the flywheel spins just that bit further before it hits the next fin. He said the one broken fin may have contributed, but the real problem is that the newer flywheel fins are
on a different angle to the old type. They are taking the pressure of the fully wound spring almost
side on instead on end on compared to the older flywheel. The new flywheels were not designed with impulse starters in mind. The angle of the fins on the new ones means they can't maintain the same amount of pressure and will fail after a short amount of time. He was actually surprised that it lasted as long as it did. He said in his experience they usually fail almost immediately or after just a few spins. So I guess this is something for other people to keep in mind when replacing older flywheels with newer ones. The thing is if I had used a recoil pull starter I would not have had this problem.
Everything always seems clearer in hindsight! Funny how we are left shaking our head and wondering "What the hell just happened?" whenever we experience an unexpected failure. Most times there is a very simple explanation.
