QuestionI recently pulled off my intake filter and found oil in the air box, but the oil did not enter the cylinders. What may have caused this oil and how can I fix it?
AnswerHi Steven,
The usual cause of oil in the airbox is over filled oil in the crankcase. Another cause is a tip-over.
Check engine oil with MC upright on level surface. Add or extract oil accordingly.
Respectfully,
Mark Shively
Combustion Blow-by Gasses
By Mark Shively
Blow-by gasses are present with all internal combustion engines. As pistons and cylinders wear, compression decreases and blow-by increases. Allowing our engines to warm up even in hot weather prolongs the life of pistons and cylinders.
Look at this image. Notice the blue color is intake air-fuel mixture; red is exhaust gasses; and yellow is blow-by gasses that slip past piston rings and valve guide seals.
Blow-by gasses are present with all internal combustion engines. As pistons and cylinders wear, compression decreases and blow-by increases. By allowing the engine to warm up even in hot weather prolongs the life of pistons and cylinders.
The same is true about spark plugs. Placing a load on the engine before spark plugs reach their operating temperatures causes fuel and fuel additives to easily glaze plug porcelain insulators, fouling the spark plugs.
Pistons are forged or cast as an oblong shape by design to allow for expansion. Pistons grow round as they reach operating temperatures. Cylinders are bored true round when new, and wear into an oblong shape from front to rear. Placing a load on the engine such as driving off to work while pistons are not up to temperature causes accelerated wear on the cylinders. Worn cylinders means loss of power producing compression and increased blow-by.
Anyways, I wanted you to see what all those rubber hoses around the carbs and air boxes do and how they work. This example is a lesson three-fold; engine longevity, blow-by, and emissions. Good stuff.
Blow-by gasses pressurizes the spaces above engine oil in the crank case, and the space above the cams in the cylinder head. Once in a while we get a complaint of oil getting into the airbox. I ask if the bike was laid down. If not, usually the oil is over-filled. Blow-by gasses positively pressurizes the crankcase and blows oil thru the vent tube to the airbox.
A crankcase vent and vent tube is missing the image below. In this engine, (Kaw KZ440 or KZ400) the CC gases are vented up to the head via transfer ports and then on to the airbox. The airbox does have a drain tube.
Per EPA Clean Air Act, blow-by emissions may not be vented to atmosphere, rather reintroduced into the engine for combustion. This affects engine performance as the power producing air-fuel mixtures are displaced by the reintroduced blow-by hydrocarbons.
