Topic: 1975 CB750 pressurized oil tank with an audible whoosh

[MoMo]

In the process of putting the finishing touches on my 1975 750K ( sn251xxxx) with a motor (sn 2501xxx) that was given to me that was supposedly taken from a 750 F I came across something I have never heard before: that is after starting the bike, letting it run for a minute or so and then shutting off the motor to check the oil level there was an audible whoosh when opeing the cap.  I rode the bike  around the block and came back to check the oil again ( don't want to ride it to far as the tach seal was leaking like crazy) and the whoosh was gone.  I started the bike today for forum member mcwilliams570 , aka Matt, and the whoosh is still there.  I installed the new tach seal but haven 't had a chance to test ride for a longer period of time.  The only thing out of the ordinary is I had to use an adapter to couple the smaller lower tank breather to the larger lower motor outlet.  Hopefully I will get a chance to ride again tomorrow and report back...Larry

[Don R]

All of the lines don't attach to the tank, check the online manual for the diagram.

[MoMo]

All of the lines don't attach to the tank, check the online manual for the diagram.

Thanks for the input, Don.  It made me realize I was venting the upper crankcase into the top of the oil tank.  As I was reassembling the bike I put it back together the way the PO had it (should have known better but I went ahead without thinking-not all that unusal in these days of 60+).   Haven't found the diagram you mentioned or corrected the vent problem but at least I realized what is wrong...Larry

[MCRider]

The upper vent from the cam cover either goes to the air, or to the airbox, depending on year/K#. Look for a nipple on the airbox, if none, then let it lay, over top of the carbs and down to the swingarm.

[MoMo]

The upper vent from the cam cover either goes to the air, or to the airbox, depending on year/K#. Look for a nipple on the airbox, if none, then let it lay, over top of the carbs and down to the swingarm.

Thanks, Ron.  I just have to find time to correct the problem. The airbox is from an F and does not have any nipples so I'll have to vent the cam cover  into the air( I intend to get a very long hose so it exits below the motor and away from the rear tire).  Normally the upper oil tank reservoir would go to the airbox so I will most likely have to also vent that into the air. The lower connection from the oil tank to the lower crankcase had to have an adapter used to connect the two different size outlets.  Any thoughts on the upper oil tank hose?  Thanks Ron and Don...Larry

[MCRider]

The upper vent from the cam cover either goes to the air, or to the airbox, depending on year/K#. Look for a nipple on the airbox, if none, then let it lay, over top of the carbs and down to the swingarm.

Thanks, Ron.  I just have to find time to correct the problem. The airbox is from an F and does not have any nipples so I'll have to vent the cam cover  into the air( I intend to get a very long hose so it exits below the motor and away from the rear tire).  Normally the upper oil tank reservoir would go to the airbox so I will most likely have to also vent that into the air. The lower connection from the oil tank to the lower crankcase had to have an adapter used to connect the two different size outlets.  Any thoughts on the upper oil tank hose?  Thanks Ron and Don...Larry

On the earlier bikes the upper tank hose like the cam cover hose, vented to the air. Both went thru a little stamped steel holder on a rear crankcase bolt, and squished down between the front of the swingarm and the engine. In front of the tire. Without the holder, maybe ziptie them to the centerstand bracket(s).

Don't know why the lower hose needed an adapter. Those nipples should have been same size, IIRC.