venting the crank case

[harold]

In the thread about $4 a gal gas, hb7 mentioned picking up some hp by the larger vent hoses that Bill used. I searched for venting and only found gas caps. Does anyone have a picture or description of what is needed for this?

[Bill Jurgenson]

sure enuf pictures on the net.
here again, my top vent, 1" cooling hose, the smaller hose from below jointing the 1":at the white delrin T-fitting.

the scouring pad in the lid of the catch tank is a sort of "airfilter" for that tank to catch the oil mist inside and keep it inside.

the stock vent goes thru the balancershaft

whose most important purpose is that of a centrifugal separator. There is little point in placing a larger hose here because the "separator" cannot be larger in ID.
OVER used the much larger hole behind the OEM vent for a second very large vent hose. That hole is for the clutch arm in the pre-XTZ versions (XT, SRX, etc). You can see thru the mainshaft bearing to the space for that clutch arm. So, OVER must have milled that cavity free from the flywheel side. I've not seen the insides of a real OVER motor so can only guess.

regardless of where the additional vent is put, it must be obvious that the separator is being circumvented and so more oil is being pumped into the catchtank or airbox as the case may be. Ideally a catchtank ought to be at least as large in volume as the displacement of the motor. And it should be pressureless unrestricted.
Old engines very often have a poppit valve in the vent housing which lets air out but is sucked closed and so helps create a near vacuum in the crankcase on the up stroke which helps to draw back what has been forced against any passways. The BSA A7 and A10 motors have a sort of rotary valve which does the same thing. Bascially that was supposed to help keep the joints oil-free. But when such engines were taken racing, those vents were replaced with a long hose out the back inot which usually went a wire brush or two to catch the oil and let it drain back. The small but very potent Motobi 250 had such a hose as well as two vents either side of the rockerarm cover.

I am convinced now that a big part of the vent problem with the Skorpion is the plastic oil tank which I also used in the racer like most everyone else. The SZR with its aluminum tank does have the problem or at least not to that extent. I never have a footbath in the airbox like I did with the Skorpion despite constant high revs.
And since making my own round oil tank for the Skorpion, I no longer have that problem with it, either. In fact the the purpose-made vent tank I use on the MZ is always practically empty.

under the seat, just visible at the right


The balance hose between crankcase and oiltank blows the plastic tank up with every downstroke; you can see it and feel it and being flexible plastic it contracts on upstroke and blows the vent gases back with the same force at least like a rubber ballon.

[DAVID THOMPSON]

the bmw /2 motors to 1969 have a rotor in the crankcase breather path
my r50s is blowing badly and needs worked on in that area
one of the projects when i retire
had it since 1975 and about 165,000 miles
so its a bit worn as some one before me rode it till it died
and then i rebuilt it..
dave

[Bill Jurgenson]

at the mileage, that can only be really seriously bad rings and/or sleeves.
Of course a BMW has the same problems as a single, perhaps worse since it is a boxer. i.e. both pistons go out a and back in at the same time, as opposed to a horizontally opposed in which they alternate and so don't really change anything with the crankcase volume.
Parallel twin are the same a single, too.
180º twins like the CB72 or the MZ1000 alternate ans so are both better balanced and also do not have the problem. The BMW is also a 180º crank so it si perfectly balanced.
with increasing number of cylinders, the problem gets smaller and smaller.