Small Engine Repair
Auto & Repair

   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 not run correctly.
You can often see if the key is sheared by just removing the starter pulley on top of the flywheel, but you may have to pull the flywheel to check
Key Sheared Off
You will have to remove the flywheel to replace the key if it is broken or damaged.
There are two common types of keys:

  A rectangular or D shaped piece of soft metal
that locks the flywheel and shaft. You should be able to see if the two identically sized slots are still aligned.

  A piece of soft metal with an L-shaped cross section
. The slot on the crankshaft is narrower than the slot on the flywheel and is slightly offset. Again, it should be obvious if the two slots are  aligned.
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