CV25s sorta backfiring suddenly and will not start

[rrodger9]

I have an older CV25s Spec # 6913 on a Ransomes Bobcat. It has a Nikki carb on it. The other day I started it just to back it out of the shop. It ran fine and I allowed it to run for a while while I did other things before I shut it off. When I went to start it again to move it back into the shop, it refused to start. It fouled the plugs with gas and it would sorta backfire without the fire and seemed flooded. I cleaned the carb but got the same result. I then purchased a rebuild kit for it and completely rebuilt the carb also wit the same result. I checked and have spark from both sides. I also pulled the valve covers and made sure everything I could see seemed to be working properly when I turned the motor over by hand. It was. Last year I converted the engine to a standard ignition rather than the electronically controlled ignition as I had a coil that went out and they no longer sold the coils that went to the electronic ignition so I purchased a kit that came with a new flywheel and a new woodruff key. I replaced both and then used the mower without any issues I pulled the flywheel bolt and can see the woodruff key and it seems to be intact and not sheared but I am not sure I could fully tell without pulling the flywheel. This has me stumped. The usual suspect would have been some dirt in the carb and that never caused the very violent backfires. The motor really shakes when that happens. Any ideas would be appreciated.