Small Engines (Lawn Mowers, etc.): Crankcase ventilation, crank case vent, crankcase ventilation

Question
Dear Sir,

  I have a Briggs Vee Twin in it's second year of light use in a Cub Cadet. It's been great!

  A week ago it began missing which was worse as rpm was increased. I found the problem was a rich mixture caused by
an oil impregnated paper air cleaner. Oil from the crank-
case vent hose was entering the air cleaner. There was a lot and the oil level was down almost to "add oil".

  The check valve is fine and it has not done it again. A finger held lightly over the end of that hose will be vibrated lightly by tiny puffs of air or vapor discharging. But no oil appears to be entrained.

  I believe the crankcase is tightly sealed and I am assuming that piston blowby is the only possible source. Is that true and is that normal for a nearly new engine with limited light usage? It is not a high volume of air or vapor coming from that hose end. The finger tip just barely vibrates.

  Will this happen again?

  That air cleaner chamber must run at essentially atmos- spheric pressure outside of the paper filter so manifold pressure is not a factor to induce air flow in that hose.
I am considering discharging that hose into a glass jar on
the firewall. The jar could then exhaust to the air cleaner chamber or down to the field surface.

  My dealer says it must have been overfilled. Maybe so! But would it then continue to down the oil level to the "add oil" mark?
piston blow-by

Answer
When oil comes up to two and into the air cleaner assembly your crankcase is not ventilating.  It is pumping oil with each stroke of the piston up the breather out the two and into the air cleaner sample.  This comes from the crankcase leak usually at the dipstick seal or the oil fill seal.  You could also have a leak were the crankcase attaches to the oil sump there is gasket there that could leak.  Air could also be entering at the oil filter.  It could be caused by piston blowby, but I don't think so.  Not unless you have not been diligent with your air cleaner maintenance.  A blowing head gasket or valve cover gasket could also be the cause of the leak.  The check valve is used for the fuel pump and it pulls with each stroke of the piston.  If the check valve is bad it could also cause pressure.  The other thing to look at would be oil and gas caused by an overflowing carburetor.  This is unlikely seeing oil level is down.  The first thing to do would be replaced the breather and the gasket, check for air leaks and see if the problem goes away.  I hope I have helped good luck on your project.