Crankcase pressure?

How do I determine that I have to much crankcase pressure on my 650L ?When the bike is idleing,it has a strong pulse of air coming through the breather hose.

That strong pulse of air is caused by the piston moving down in the cylinder bore- just about 650cc worth of air! At higher revs, the piston moves down and back up faster than the air can leave via that little hose, so the pulses smooth out a bit.


I understand what it is,but is it to much,is it blowbye?

is it smoking out the breather

No,not smoking

Its normal mate.

Leave it alone and ride

if it wasnt there - you would blow you cases apart on the first creek crossing

Its normal mate.

Leave it alone and ride

if it wasnt there - you would blow you cases apart on the first creek crossing

I know its normal,but what I'm asking is how much is to much?

If you're not getting a bunch of smoke or a lot of oil, its normal.

If it goes beyond normal, the combustion gases leaking past the rings will cause a lot of smoke, probably cause the bike to overheat, and will carry a lot of oil out the breather.


You would be amazed at how much technical information there is on the web about crankcase pressure, especially for racing applications. However, your question has a fairly simple answer. The piston going up and down causes the pulses that you feel. If it weren’t for blow-by, a breather wouldn't be necessary. However, because blow-by does occur, more or less depending on how good your rings are, pressure would rapidly increase in the crankcase if there were not a breather hole. It would increase to the point where oil would be pushed past any weak points in the engine and you would have really bad oil leaks. On the XR650L, the breather outlet is 1/2 inch but is reduced at the "T" fitting to 3/8 inch. I am sure that this breather size is designed by Honda engineers to allow for the correct crankcase pressure, which is probably atmospheric pressure plus or minus (thus the pulses). To find out precisely, you would have to find a device that could be put in the crankcase to measure pressure while the engine is running and have some engineering data from Honda to compare the results to. The bottom line is, pulses are a good thing. Not to worry.

Hey, I am no expert; I have been reading a lot lately on the same question.


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