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Mend Briggs & Stratton Lawn Mower Engine Carb

BRIGGS&STRATTON 12HP STARTS,CARB FILLS WITH OIL AND STALLS?

MY BRIGGS & STRATTON 12HP,SIDE VALVE STARTED FINE AND RAN FOR ABOUT 1 MINUTE BEFORE THE BREATHER FORCED ENOUPH OIL INTO THE CARB TO STALL OUT.ENGINE FEALS LIKE IT HAS LOTS OF COMPRESSION.I AM ABOUT TO REMOVE CYLINDER HEAD AND INSPECT.ANY HINTS ON WHAT I MIGHT FIND?

MARTIN
June 2009

Briggs & Stratton Garden Tools, Lawn Mowers Lawn Mower Engine Carb

First of all, check for overfilling the crankcase with oil. The capacity is 3 pints after completely draining. The oil dipstick on these engines can fool you. I removed it and went back to the older style removeable plastic thread-in plug on my 12 and 12.5hp Briggs engines.

Next possibility is plugged drainback holes in the crankcase breather assembly. Installing the gasket for the breather 180 degrees wrong will also block its oil drainback holes.

While you have it apart, check the valve clearances, intake should be .006 and exhaust .010. The intake valve on these engines will get tighter as it wears.

You can download an exploded view .PDF parts manual for your engine at Briggs and Stratton's web site.

haywiremechanic
June 2009

Usually excess oil in the carb is simply due to the oil level being too high - the breather pipe at the back of it is really meant only to deal with fumes, and if there is too much oil around, this gets forced up the pipe instead.

John
June 2009