Motorcycle owners who ride the older style Evo models may gone through the experience of oil blowing out through the air cleaner when they start the bike up after letting it sit for a long period of time. And once the bike is ridden and brought up to running temperature, or if it's ridden on a daily basis, the problem seems to go away.
This is not an unusual occurrence with the older style Evo engines. But even though it may be a common thing, it's not normal for this to happen despite what you may have been told. The first thing to understand is that the oiling system on these motors is what is known as a dry sump. What that means is the oil supply is located in a different place away from the motor as in an oil tank. Because of this setup, only a small amount of oil should ever collect in the crankcase (sump) of the motor at anytime. When oil starts squirting out through the air cleaner, the motor is experiencing what is known as "wet sumping" meaning too much oil is gathering down in the crankcase when the bike sits for long periods of time.
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The reason this happens is because the check valve (a small ball) in the oil pump isn't seating properly. When the engine isn't running, oil will seep past the unseated check valve and drain into the crankcase (the sump). When you start the bike up and pressure starts building along with slinging the excess oil around, it's gotta travel somewhere. So out the breather it goes, into the air cleaner and onto the floor before it gets a chance to get pumped back into the oil tank.
There are a couple of reasons that the check valve may not be seating correctly in the oil pump. Some dirt, metal filings or other debris might be on the seat of the valve or on the ball. In some circumstances, metal may have passed through the oil pump and maybe caused some nicks on the ball giving the oil an opportunity to leak through.
To fix this situation, the oil pump needs to be removed, the ball valve and spring taken out of the oil pump housing and all the components given a good cleaning. After cleaning the components, carefully inspect the check valve parts for any damage. In fact, you may want to use a magnifying glass so you can get a real close look at it. Once you've got everything all cleaned up and you see no damage, put it all back together, reinstall the pump and see if that takes care of the problem. If this problem continues after you've cleaned and inspected everything, chances are you'll need to replace the oil pump.
One more thing: When you do leave your motorcycle sitting for long periods of time, check the oil level before you start it up. If the level is low, don't add any oil until you run the motor for a little bit. The reason is because the motor may have "wet-sumped" while its been sitting and a good amount of oil may have leaked into the crankcase. And even though your dipstick may indicate that the engine oil supply is about a quart low, that missing oil may be sitting in the crankcase. If you add a quart of new oil before you start the bike up, you'll overflow the oil tank. So it's a good idea to let the motorcycle run for a few minutes to circulate the existing oil and then check the level.
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