13.5 OHV B&S won't stay running, blows gas out carb.

I just bought this riding mower, used (mistake #1) from an individual (mistake #2) and paid too much (mistake #3).. I ran it once, it seemed to work well enough. two weeks later, it started out fine and after about 20 minutes, blew a huge blue/white smoke out of the exhaust and no longer developed any power. I looked at the mower while the air cleaner was off and trying to keep it running with the choke mostly closed, it is VERY load pulsing noise from the carb and blowing gas out of the carb. It wont stay running like this long, even manually working the choke and gas feed. The seller has vanished. I bought this mower, because my Toro needed $400 in repairs, muffler and a new sump ?? says the mower shop. This used mower cost $500, thought I was better off :-( Well, now I have two broken mowers and very tall grass! Help!! What could it be? Something repairable?