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Discussion Starter #1
So I was changing my oil, and after filling the bike with new oil, I started it up to circulate the oil in order to get an accurate reading through the sight glass. I had left the oil filler hole plug off and noticed that there was a strong blast of air coming out of the hole as the engine was running. When I say strong, it almost felt like the air coming out of the exhaust! I could feel the air pressure coming out of the hole a full three feet away.

Is this normal or not?? I bought the bike a couple months ago and it seems to run just fine. Starts right away, and it feels like it has plenty of power(this is my first RC though, so I really have nothing to compare it to). Bike is safety wired, so I'm sure it has seen some hard riding, and I do believe it has the smog equip removed/blocked off(not 100% sure though, just assuming because all the other free mods are done on it.) Sooooooo, is this a sign of tons of blow-by and imminent engine failure, or is it normal?
 

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Remember that these twins have two big pistons with rods connected along the same crank-pin center. So you get two pistons that travel up and down only 90 deg apart.
So when they are on their down strokes, you get a briefly pressurized crankcase.
Normally, the pressure goes up the the crankcase vent and through the crankcase vent breather tank where it catches the oil mist from the air.
That's why every so often, you have to drain the hose from the breather tank.
 

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Discussion Starter #3
Ok, that makes alot of sense. I didn't realize the pistons are staggered only 90 degrees apart on the crankshaft. Now I am understanding why when I searched this subject all I could find was methods of venting the crankcase more adequately. Does better ventilation sound like something that would be worth exploring?
 

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Of course as better crankcase venting is gained horsepower.
You could check out the Krank-Vent product for instance.
 

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It may be a give and take situation. Excessive crankcase pressure may slow down the power stroke but it also helps the exhaust stroke and compression strokes. It also assist circulation of returning oil back down to the pan when G forces may act against oil. In a way it stabilizes the speed of the engine while a freely breathing engine accelerates and decelerates more. V-engines seem more sensitive to this and singles are even worse, while a 4cyl creates no volume imbalances.
Two strokes work as isolated singles and use this crankcase pressure as part of their compression.

Note: in engines where gas pressure is part of the oil circulation system you may be hurting it by allowing it to vent.
 
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