Crankcase Breathing Issue

[J-Dog]

Hi-

I have a somewhat modified CB750.  It has been Cafe-ed, shall we say, before I bought it, which resulted in the removal of the air box.  Lately it has developed an interesting problem.  When the bike has been run even for only a few moments, the oil tank will become pressurized and the pressure will escape with a slow hiss.  When I ride for a few miles, sometimes oil is forced out around the cap by the pressure, and when I stop and open the cap on the oil tank the pressure nearly forces the cap out of my hand.  I know this can't be normal.  This is an older engine, I believe a 1973.  How is it supposed to ventilate the crankcase?  Is there a breather somewhere which I can't find, which is clogged?

Thanks!

[Re-run]

let me check on this but I think you just need to change the breather hose out. When you have a bored engine, more pressure is created and if you change out the stock breather hose for a larger one with a filter on the end, it usually relieves this pressure.

[J-Dog]

Thanks for the response!  So exactly where might I look for the breather hose?  I know where it is on the late-model engines, but I can't find anything resembling it on this engine.  I read in one forum that on the very early engines it ends in the front-sprocket area, under the case-cover which hides the sprocket, but I can find nothing in a manual about this.  Any clues?  Was it intended to drip oil onto the chain, as in some old Brit bikes?  Or did it come up to the breather box, before the box was removed?  And if it did, where did it come from?

Thanks!

[Bill]

Sir,

I'm a newbie to the CB750 world, but not to engines (drag racer).  May I suggest that the crankcase is becoming presurized from excessive blow-by.  I think you need to bore it.

-bill