The lawnmower will not crank, has too much compresssion. The engine will not turn over.

Probably set piston at 1/4" past TDC (power stroke) and 0.004" clearance for each valve.

To find TDC use a dial indicator in the plug hole and make sure the valves are closed (rockers are loose) are loose.

Watch for worn cams on some of Briggs engines. Compare the max intake and exhaust lift.