Hello,
I have a 15.5 Tecumseh riding mower. Last summer it had gotten where I would have to turn by hand the fly wheel screen to bypass the compression stroke so the starter would crank the engine. Before I came to this trick I replaced the battery then later the starter with little to no effect. Now this year 07 even rotating the engine by hand does not do the trick any more? Some have indicated that there is a bypass valve that lets the engine start with lower compression. What can I do? Thank for any help.

Remove the spark plug, and see if it cranks easily that way. I suspect an electrical problem, as I think you did as you replaced batt and starter. How old is this mower?
Frank

Yes check the compression relief valve,and make sure all your electrical connections are good