White smoke is unburned diesel fuel. You probably have an injector that needs rebuilt.
How many hours on the engine?
You can get a completely rebuilt engine with warranty for about $2500. The one I found was for a B7100.
https://countrysalesandservice.com/kubota-d750b-b5200-kubota-tractor/
But you probably don't need to replace your engine and doing so before you find the CAUSE of the symptoms you describe would be foolish. Injectors can be had for as low as $44 each; website lists OEM part number but that doesn't mean they are OEM. I doubt it, at that price.
https://www.tractorpartsasap.com/pa...s/kubota-tractor-parts/b7100/fuel-system.html
You can get a completely rebuilt engine with warranty for about $2500. The one I found was for a B7100.
https://countrysalesandservice.com/kubota-d750b-b5200-kubota-tractor/
But you probably don't need to replace your engine and doing so before you find the CAUSE of the symptoms you describe would be foolish. Injectors can be had for as low as $44 each; website lists OEM part number but that doesn't mean they are OEM. I doubt it, at that price.
https://www.tractorpartsasap.com/pa...s/kubota-tractor-parts/b7100/fuel-system.html