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Doug E
Doug E, Mechanic
Category: Small Engine
Satisfied Customers: 2887
Experience:  30 year expert on John Deere and Kawasaki products.
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can I blank crank case breather on 280cc 2 stroke Rotax Aprilia

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can I blank crank case breather on 280cc 2 stroke Rotax Aprilia trials bike

What exactly are you trying to do? What problems are you having?

Customer: replied 4 years ago.

hi Doug,I am wanting to blank the crank case breather off,because all it does is fill the air filter box with 2 stroke mixture.The pipe from the crank case has a one way valve and then is into the air box.I am sure earlier rotax engines used on SWM trials bikes ran fine without this system.Also I thought the oil in the fuel mixture was to lube the bottom end,so why would it be fed back into the carb inlet. Hope this helps, look forward to your reply.


Thank you for that information.

There should not be any gas in the crankcase. If you do have gas mixing with the oil then there is a problem. What year is your bike? Is it the carb model or EFI? If it is a carb model then the float valve in the carb is either set too high, it's worn or the rotary valve is leaking on the engine. If it is an EFI model then the injector is sticking or the rotary valve is leaking on the engine.

Once the fuel problem is repaired, you will no longer have gas in the oil and that should solve the problem of gas/oil getting into the airbox.

You can't plug the vent because it will cause the seals and gaskets to fail on the engine. You can install a oil catch tank in the hose between the engine and airbox to capture any oil that escapes. Being the bike sometimes operates at crazy angles, that would be a good thing to have. Here is an example of one:

I use those on most of my race motors to catch any oil that vents out. They make them in different sizes and shapes and you should be able to find them for $20-$30 on ebay. The chinese scooters need them because they vent oil all over the place without them.

The first thing is to solve the gas problem and be sure to change your crankcase oil to get the gas out of there. Let me know what year you have and I'll know more about what fuel system you are using.

Doug E and other Small Engine Specialists are ready to help you
Customer: replied 4 years ago.

Doug, thanks for that. I am confused, or may have missled you,but the bike is a 2 stroke and does not have oil in the crankcase it only has oil in the gearbox.The bike is 1985 model and runs a dellorto PHBS 26 carb.

My mistake on that! I'm sorry for the confusion there.

Are you getting gas into the gearcase? If so, the rotary valve would be leaking. Being it's a 2-stroke that would be the only way to get gas into the gearcase.

The rest of the information should be ok. You can't block off the gearcase vent because it does build up pressure and can case a seal or gasket leak. Putting an oil catch bottle between the gearcase and airbox will keep oil out of the airbox. You don't want that oil getting into the engine because it will not burn properly like your 2-stroke oil would.

Are you getting gas into the airbox? You mentioned a gas mixture in the airbox so that's what got me confused.