Gas from the breather hose
 

Have a 96 Yamaha that had a stuck float valve. I changed the carburetor and it runs for about 30 to 45 seconds and then dirty gas starts to shoot out of the crankcase breather hose into the air filter box.

Gas in crankcase.
 

Did you change the oil when you changed th carburetur? The crankcase may be
Loaded with gas from the old carb.

Haven't done it yet, but that is what I suspected. Could there be another problem to look for should changing the oil not solve the issue?

I will do that next. Would anything else be something to look for?

I should solve the problem.