what causes gas to get into the crankcase;12hp briggs stratton motor?

Hey, I've dealt with this problem a lot, it's not just Briggs I have a 18hp Kohler. One day after I parked mower in garage I woke up went to the garage and saw my full tank of gas was gone and concrete was stained, I simply fixed it by cutting fuel line and inserting a gas shut off valve haven't had a problem since, it's been 2yrs now, hope this helps and good luck.

Sizemore
July 2017

I had that problem last year, and have it again this season with my 9hp Briggs. My Snapper dealer tells me that the additives in modern fuel are damaging the carb needle, thus preventing the fuel from cutting off and allowing it into the crankcase. I replaced the needle and seat last year and had one mowing season without the problem. He tells me that this is an ongoing problem and he has seen the needle go bad again in as little as two weeks and there is no current solution other than needle/seat replacement.

If you replace the needle and seat yourself, make sure you completely drain the crankcase of fuel/oil. Fill with new oil, drain, and repeat. I turned my crank a few times with the first oil change, not starting the engine, but to rotate the parts into the new oil, then drained it all, and refilled. Fuel breaks down the oil and you want to make sure you get as much fuel out of the crank possible.

Replacing the needle and seat (with a needle and seat kit) is a pain, but much cheaper than paying someone.

Clayton Adams
March 2008

Possibly tank not venting properly - see: http://www.ytmag.com/cgi-bin/viewit.cgi?bd=gardent&th=51882
(or http://tinyurl.com/256bue).
Gas getting in oil when motor is not running is usually caused by the needle and seat under the float in carb not shutting off the fuel flow. See:
http://www.askanowner.com/qa/view.asp?guid=&sid=1&qid=156726
(or http://tinyurl.com/ypc95g).

Jerry
March 2007