I have a 2001 Murry lawn mower that has fuel in the crankcase. It's a 12.5 HP Briggs & Stratton model 28V707-1113-E1. Lots of people say it's the needle valve in the carb, but I'm not sure that's the case even though I could be wrong. If I let it run for a while say 30 seconds the fuel/oil mixture will flow past the breather assembly up the tube and shoot out the air filter assembly like a water fountain. So tell me what you think thank you.

Well first off do not run the engine with fuel in the crank. Remove the bowl and see if you can totally shut off the fuel by raising the float, not one drop. If the needle drips even a little you have found your problem, needle and seat. Clean or replace.