This mower made an awful racket when it started from the top of the
engine. Right away I suspected the starter ratchet. Usually
you can get the top cap off of a B&S engine by taking off three bolts.
This is an unusual ratchet for a B&S engine (more typical for a Tecumseh)
and when I disassembled it I found one of the pawls stuck out. All
it took was loosening the center screw and putting a little peg on the
pawl back in a slot in the cap.
Usually B&S engines have a metal shaft with a ball bearing clutch on top of the flywheel. The ball bearings often get stuck, or the shaft sticks to an inner shaft, particularly on engines that have been left outside.