Yard Machine Riding Lawn Mower

a new start and new spark plug, cleaned carb.

If it starts, then dies, it is probably running out of gas. Try cleaning or replacing the fuel filter online.
If this does not work, it may be the float sticking in the carburetor. Get a plastic or wooden-handled screwdriver, hold it by the end of metal, start the engine and use the handle of the screwdriver to gently tap on the side of the carb. Sometimes it works, but you'll probably do it every time you rebuild the carb.
And, best of luck - I know the frustration of these things can be.

Is it oil? The air filter dirty? Is there enough fuel? Have you any bottlenecks?