> We have a 1946 B Model John Deere that leaks fuel into the crankcase.
> Any ideas on this problem?
Yep, I know what is happening. Your fuel shut-off valve is not working very well, and neither is the float-valve in the carburetor. Gasoline has a very low surface tension and a low density and it can get through the TINIEST little "openings". At night when you are sleeping, gas is leaking past the carburetor into the intake manifold. Then, silently as a mouse, it is creeping down past the intake valves and into the cylinders. Just one drop at a time, it is sneaking past the piston rings and tiptoeing its way into the crankcase.
What Johnny Popper needs is a NEW fuel shut-off valve. You can't rely on the float valve to do the job; that's WHY they put a shut-off valve on there.
Now, it is quite possible that your shut-off valve is just fine, but perhaps you have NEVER been shutting it off. Let that be a warning to EVERYONE: Even if you believe your carburetor is working properly, always shut off the fuel control valve when stopping the tractor for an extended period of time. If you don't, you may wind up with 5 gallons of gas in the crankcase if the carburetor float happens to stick.