Joe: I have a 5hp Briggs model 130232 that I overhauled. It will fire, but as soon as it does it shears the flywheel key. My timing is off I think.
First thing, make sure the crank and cam are in time. If that is good, clean the tapered flywheel mounting surface on the crank and in the flywheel with alcohol, then torque the flywheel to 60 ft.lbs. If there is any oily film on either of those surfaces, the key will shear upon start up.