Engine turns over extremely hard, have to use heavy jumper cables on my truck and a battery jump box, both, to get enough juice to turn it past the compression point. I am on the 3rd starter... starts and runs fine if I can get enough power to turn it over. Any suggestions? It's a briggs model 31p777 type 0348 e1, code #040520za... 18.5 hp. Help!!

I would check my valve clearance first,
It should be 004-006
Possible Camshaft problem.
But check the simple stuff first

Check the oil (probably mixed with gas) level, needle valve bad causing gas to enter crankcase.
You will need a new needle valve and seat, a new oil filter, and fresh oil.

Hi,
Just an addition to give you that (in)famous YouTube link:
YouTube - Lawn Mower Repair Valve Adjustment (http://www.youtube.com/watch?v=NjzX-PUjKmo)
It will look a bit different, but the procedure is the same.
Peace,
Clarke