Engine Eating Oil Through Crankcase Vent

Frank.WI.

9 years ago

Engine: B&S 16.5 Hp OHV Single cylinder (Sears riding mower)

Two years ago, I did a major overhaul of the Briggs & Stratton engine on my Sears riding mower. I knew a new oversized piston was needed, but when I pulled the engine apart, the head was cracked. So, in addition to the new piston/rings, I purchased a complete new head, with new valves, etc. The engine ran like a champ for the last two years.

This week, the engine was running fine for about 20 minutes. Then it started running so badly, I went back to the garage so I'd be near tools. I pulled the spark plug and it was wet with oil, gunked up with carbon, etc. Up to this point, I hadn't been using oil. I quickly pulled the air filter housing apart and there was a fair amount of liquid (not a film) oil in the housing. It was obviously coming into the carburetor via the crankcase vent.

I put in a brand new spark plug and 20 minutes later, the engine started running rough again. Back to the garage. Spark plug gunked up again. Pull the head, valves look good except for the carbon deposits. Pushrods are ok, pushrods are moving as required, valves move fine. Nothing of note in the cylinder area. I can't see any cracks or other defects that would account for pulling a LOT of oil via the crankcase vent to the carburetor.

I'm stumped. I took the crankcase vent assembly out and soaked it in mineral spirits, swished the spirits around, and poured it out. Vent assembly appears to work ok, easy to draw air through in one direction, slight back pressure in the other direction.

Is it worth it to replace the crankcase vent assembly? It seems to be working.

Thanks!!!

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Comments (7)

gator_rider2
9 years ago
If air filter gets oil element need new filter may need re-clean vent again. new filter easy get oiled again. Crankcase will get higher vacuum with dirty filter that lift oil up.
bill_kapaun
9 years ago
Check the oil for the presence of gasoline.
mla2ofus
9 years ago
X2 what Bill said.
Mike
tomplum
9 years ago
So do you know where these headgaskets typically fail and how to spot them? Certainly a failed head gasket can cause this condition. Carbon deposits = oil burning.
walt2002
9 years ago
Sounds like typical head gasket failure, check on the block side between cylinder and valve guide gallery.

Walt Conner
optsyeagle
9 years ago
In this picture I had a blown head gasket. You can see the breach area at around 2:30 o'clock. Mine was a slight breach visible on the gasket side that faces the cylinder. My mower ate about 2 ounces of oil per hour, a lot less then your but perhaps your breach is much larger. Certainly look closely at that head gasket, as the others have suggested. With my new gasket, installed in the spring, my mower ate absolutely no oil this entire season.

Sniffing your oil dip stick for gas is another possibility. I would suspect that if your carb is leaking gas into the combustion chamber and then into the oil, that your mower would also take quite a few cranks to start. If it does, then flooding could be your problem, if it doesn't then I would suspect the head gasket.

By the way. My tractor is also a 16.5Hp B&S made in 2004. If it is a gasket breach let us know and we will give you the upgraded torque values and sequence to keep it from happening again...or at least delay it somewhat.

This post was edited by optsyeagle on Tue, Sep 16, 14 at 11:50
Frank.WI.
Original Author
9 years ago
Thanks to all of you who replied to my post. The Craftsman mower starts and runs like it is brand new. The head gasket had a chunk missing between the valve side of the head gasket and the cylinder part. In addition, the valve lash was way off. I can't believe that a few thousandths of an inch on the valve lash would make such a difference in starting. I rebuilt the mower about two years ago and when I started it the first time, it tried to start and then stopped on the high compression point. I figured that the inability to start was because the new piston/cylinder was so tight. The head had to be replaced as it was cracked. I didn't realize that the new head assembly would need to be adjusted as it came complete with new valves, springs, etc. For two years, I've been pulsing the starter so the engine would rotate backwards so I could get a "flying start". The increased momentum allowed the engine to overcome the compression and start. Now it starts like it was a new engine.

THANKS!!!!