I have a Toro Zero turn 4200 with a 16 hp Briggs and Stratton motor. I have had the mower 4 seasons. When I try to start it the motor gets to the compression stroke and will not go any further. Sometimes if I turn it back and try again it will go past, pick up some momentum and fire up. I am not sure what to check or where to start.

Places to start - in order.
Battery voltage.
Starter condition.
Flywheel timing.
Valve adjustment.

New battery
Starter turns the engine over
Not sure how to do the two others.
Once it starts, it runs fine.

Flywheel timing - look for a misplaced sheer pin. Shaft to flywheel. That will be obvious. It happens when you hit something hard.

Valve adjustment might be difficult. First you need to find the specs. Repair manuals are available at Home - Briggs & Stratton (http://www.briggsandstratton.com) (Pay). Exploded views and parts lists are free. The model and type are STAMPED on the blower housing.

In general, the procedure is to move the piston so it's 1/4" down in the power stoke and adjust both valves at the same time to the proper gap. Feeler gages are used to check the gap.

Sounds like you may have dinged the shear pin getting the motor a little out of time.

I live in a cold weather winter area. When taking the mower out for the summer at first there was a slight turn of the motor, then just clicking and finally no clicking. My voltages was 10 and under on the battery I replaced the battery and everything started up fine. I found out that during the winter the battery water will freeze if you don't keep is charged. Hope this helps someone else.

-E

I'll bet you backfire a lot too. I had the same thing occur. There was a factory fix for fuel leaking into the cylinder. Didn't work though. I installed a little shutoff valve in the fuel line as near the float blow input as I could. When I parked the mower, I turned off the fuel supply to the carburator. Took care of the problem. (the reason your battery strained to turn over was because of all the fuel leaked into the cylinder creating too much head pressure... Backfire city).

I think KISS is right,
If the valve clearence is off then the compression release doesn't work and will not go past the compression stroke.

Hi,
I will simply point out that to date, manuals are online at:
K&T Parts House Lawn Mower Parts and Chain Saw & Trimmer Parts (http://www.mymowerparts.com/pdf/Briggs%20&%20Stratton%20Service%20and%20Repair%20Manuals%20S pec%20Charts/)
Peace,
Clarke
PS It is the valves

The sump is full of gas/oil--the needle valve in the carburator failed to shut the gas off from your fuel tank and the sump is full of gas/oil mixture.
Drain the oil/gas from your motor --refill it with new oil --motor will smoke when first cranked--let it run for a little while this will clear up.
Use the manual shut off valve to shut the gas off after using the mower each time you use it.