I try to start my john deere riding mower and it doesn't want to start and will back fire.

Old gas and water in the gas are a likely suspect. Loosen the bottom carburetor nut until gas begins to leak.Try to catch it all in a short clear glass. Any water in bowl will come out first and be evident on bottom of the gas, which follows, and more coming from the tank. If some leaks on the frame the water will appear as bubbles rolling off through the gas. You only need an small bit to see if water is on the bottom of your tilted glass. If so screw it back and try again. Good gas is almost clear with a yellow tinge.
Bad gas is much darker and maybe an orange tinge plus can have a varnish paint smell. If so - safely dump it and the tank gas. Rinse the tank with fresh gas and start all over.
I have to do this with my tiller often and redo when it quits because some water was still in the tank. LoL

Hi,
If it has a Kohler engine, then it is most probably the flywheel key is sheared.
Peace,
Clarke