Motorcycle Repair: Oil in the air box, combustion gasses, engine crankcase

Question
G'day Mark...

Just to let you know my SRX is running smooth...no worries of late with that ride, now it's my nephew's I'm writing about.

2005 Kwaka ZZR-250.  Brand spanking new and following its first service it seemed to develop an oil leak on the left side near and above the kickstand.

I checked it out and notice the oil level, when on the centre stand, was way too much, with no visible bubble in the inspection window.  I told him this and he took it back to the dealer who said it was a tad full and promptly drained some.

But, that being said, the other day he undid a clip on a hose on the right side of the bike and drained about 50mm of oil from what he said was an air box drain line.

Last night we took the top off the bike, checked the air box and cleaned air filter and box.

So, question to you...what is causing the return of the oil...or is it just residual from the first instance?  Anything else I should check-e-mail me at bill.murphy@health.wa.gov.au

Many thanks mate...love your work!

Bill

Answer
Greetings Bill,

Oil found in the airbox is related to the cranckcase being overfilled with oil.

Blow-by combustion gasses pressurizes the space in the engine crankcase. When oil is overfilled, oil may be forced through the crankcase vent and carried into the airbox via pressure vapor.

This is also common when carbs varnish. Petrol overfills floatbowls flows into carb Venturi's, cylinders, and into crankcase. Petrol saturates oil and causes ccrankcases to overfill.

Thanks for the compliments.

Respectfully,
Mark Shively