Had the same problem with my MTD Briggs/Stratton push lawnmower. Took it to a lawnmower repair shop. Brought it back home after the tuneup and the mower still ran fast, then after mowing a while it would stop. Took it back and they installed a carburetor kit.
http://www.buyhardwaresupplies.com/?t=5&m=g1&itemNumber=7030372
http://www.youtube.com/watch?v=HDQ0elcCOlY&feature=related
http://www.youtube.com/watch?v=yI10s67qvFY&feature=related
http://www.youtube.com/watch?v=RJ0qPDYwQ_w&feature=related
Greg
February 2009