Having a little problem with a 21 hp briggs and stratton ohv craftsman mower model 33m777. It had a blown head gasket and was spitting oil out of exhaust with alot of fuel in the oil. I replaced the gasket and it ran fine for about 10mins then the oil filled with gas again. I replaced the carb thinking it was the float and needle. Same thing ran for about 10mins before filling with gas in the crankcase. Took the head off and and reseated the valves and and adjusted them. It ran fine for about 20mins out mowing then came back into the shop to check for leaks and the breather tube had come off and was blowing oil out of the tube and the oil was filling with gas again. Going to replace the breather and filter, but cannot for the life of me think that will possibly fix it. I am stumped and just down to rings, but the mower starts and runs fine when the oil is drain and refilled to correct levels, no smoking or rough running until gas fills into the crankcase. If anyone has any clues would be appreciated.
The engine has a vacuum fuel pump which seems to be pulsating when the engine is turned over. Just taking the line off the vacuum pump to carb and watching it as i turn the key seems good. Can check the carb again which is brand new, but still may be the problem.
The only way for gas to get into the engine is either through the carburetor or the fuel pump. since the engine is running fine, it would appear that it is not running too rich. It would really need to be running very rich to flood the crank case. So much so that the engine probably would not run.
This leaves the fuel pump. since it is a vacuum pulse, it gets its pulse through the crankcase. if the fuel pump had a leak in it, this would more than likely do it. I do not believe there is a check valve inline (at least one that is replaceable).
Save the old carburetor and replace the pump and it should be good to go.
Did you check the head & block for flatness when you did the head gasket ?
The head mounting is a fundamental flaw with this engine.
It really needs a bolt in the pushrod tunnel.
The casting goes hollow there so the engine runs fine till it gets nice & hot then leaks under the gasket.
I do 5 - 6 intek head gaskets annually and only 1 will be ok to replace without lapping head and/or barrel.
When I first started doing mowers I was not aware just how bad this design was and treated then the same as a vintage motorcycle.
I did too many a 2nd and to my shame 3rd time.
Now Inteks get a leak down test as part of the standard servicing so I can pick up leaking gaskets early before too much errosion occurs.
Currently I am experimenting with solid copper head gaskets which are holding up much better then the very soft B &S composite gaskets.
I didnt check the block for flatness or head either. I have ran the mower with the valve cover off and seems to not be leaking at the rocker rods side. Have a new breather and air filter coming in the morning. Going to put it back together and see how it runs. Going to replace the fuel pump to and see. The head or block could be warped since it had a blown head gasket. Ill keep updated and hoepfully it might fix it.
They don't warp usually but the leaking into the pushrod tube errodes the tiny bridge between the pushrod tunnel and the cylinder.
The actual layout is almost identical to the Vanguard singles which almost never blow gaskets however the bridge on them is about 1.5mm thicker and the bolt layout is better.
Very good information ty. Ill run it till get gets hot and check that bridge there. That was were the head gasket was blown. Last time i didnt see any leaks but was to hot either. This mower has been giving me the fits. Oh well good learning experience i guess.
