My 6.25 horse Tecumseh lawn mower starts right up when I begin to mow the lawn. When I turn off the mower to empty the grass I cannot get the mower to start. Gas is new, replaced spark plug also. Need some advice.

I had a Sears mower that did the same thing and I did the same things you did. Mine had the lever that you had to pull then crank. When this handle was released a cutoff switch would shut down the mower. I found out that after it aged, corrosion built up and the cutoff switch would not release. It was such a job to get to the switch I just took the mower to a repair shop and he picked it up and dropped it, the switch released and it was no charge for repair. Since then I pick it up and drop it when it does it. Maybe one day I'll take the time to repair it.