Topic: pressure in crankcase

[Robert]

 Hoi,

Someone knocked over my parked bike a few hours ago, however it fell to some degree softly into the grass and leaves so there seems to be no real harm done. (fell on right / brake lever side)

Of course it did loose lots of fuel (tank was full) and I'm afraid oil too.
Now I noticed on checking everything that - with the bike running and the oil dipstick thrown out - there's quite some air blowing out of the opening where the dipstick is supposed to be.

I'm still to embarrassed to remember if this is normal, I do know that there has to be some overpressure in the crankcase, but as I said, it seems quite a bit, almost as if you hold a ventilator against your face. Please calm me down on this.

[dusterdude]

robert,dont worry about the bike right now,find the sob that did it and shove a 2x4 up his ass.

[MetalHead550]

Damn straight dusterdude!  Is it a 650?  Are they dry or wet sump?  Of course they create pressure which is releaved by the vent at the top of the valve cover.  But thats at the top of the engine, so I can see your concern...

[Jonesy]

Not sure about your bike, but the 750 has a vent hose going from the crankcase to the oil tank.

[MetalHead550]

So....if your bike is dry sump like the 750 and it sends the crankcase pressure back to the oil tank(its a closed system, unlike the wet sump 550) what your experiencing is normal?  I vote yes?

[Robert]

Hi.  Yes when I find the perp he might loose some liquids too. 

Sorry, I forgot the important bits, MetalHead550 is right, a CB650 it is, wet sump, a breather hose is located at the top of the valve cover (sends pressure to air filter box).

[TwoTired]

I can't recall ever running an SOHC4 with the dipstick removed.  You'd think in 30 years of ownership, I'd have tried that at least once.
Anyway, the backside of the pistons face the crankcase and they are going to move air when going up and down.
I think I would check your breather system to see if there is any blockage.  On the Cb550 there is a foam filter that can clog up with water and oil.  I'm not sure how or if the Cb650 is different.  But, the breather hose and what not is going to offer more airflow resistance than the big open hole for the dipstick.  It will breathe wherever there is the least resistance.

Oh, and you're not SUPPOSED to run the engine with the dipstick out, you know.  Anyway, it's not a normal run mode, for sure.

Cheers,

[Robert]

Thank you all.
I checked breather and air filter element and found both good.

Oh, and you're not SUPPOSED to run the engine with the dipstick out, you know. Anyway, it's not a normal run mode, for sure.

For sure, additionally to any negative effects this could have on the engine (can't see any, but what do I know) or the environment, one might create problems that just don't exist 

I'm sure you are right on this,  it's really the pistons moving air back and forth, despite the fact that they are moving in opposite direction by pairs, they still could create some kind of pulsation (pos./neg. pressure waves) due to the inertia of air/oil mist.
The absolut positive pressure in the dipstick hole that my face 'measured' propably makes sense if one takes the hot gases escaping via the piston rings at the compression and exhaust strokes into the equation.