How do you check timing on briggs engine?

To check the timing, you will have to take the blower housing off, and remove the flywheel nut. There is a key that holds the flywheel in place with the crankshaft. If this is a riding mower, it would be a very rare occurrence to shear the key.

More than likely, you have a valve problem. If OHV the valves may need adjustment. If not, you may have a worn lobe on the cam.