I have a tractor mover with a Briggs & Stratton 15hp, it runs good at lowest speed only, unless I put it at full speed and full choke, it backfire out the muffler and shuts down at any other speed.

I ran through the carb, seems clean unless I missed something, the float was stuck and it would not start when I first worked on it, It was setting a long time in a garage with gas and oil in it, when I started working the gas was gone.

Carb can look clean... but the passages can be clogged... I'd recommend getting a rebuild kit, tearing the carb down, soaking it in carb cleaner overnight and blowing all holes out with compressed air.

Take the carb back apart and soak in carb cleaner to remove varnish. Check the position of the float. On briggs it should be parallel to the base when the carb is inverted. This is important .