If this is a Briggs engine, the correct position to adjust valves is with the piston DOWN 1/4" PAST TDC of the COMPRESSION STROKE. There are 2 times in the 4 stroke cycle when the piston is at TDC but only one of those is where you check/set the valves.
Now, you say the engine "locks"............does this mean it does not spin with the starter?
Can you remove the spark plug and turn the engine OK with the plug out?
Is this something that occurred suddenly? or has it seemed to be getting worse with time?
Look on the engine and get the 6 digit model # and 6 digit type # and post those so we know exactly which 18 HP OHV engine you have.

Don't rule out a weak battery. How old is it and does it have a full charge.

My Kohler did the same thing, with properly adjusted valves, until I replaced it.