Topic: Pressre in the 750 oil tank

[toma]

70 CB750 lets out a puff of vapor if you remove the oil tank cap with the engine running or shortly after shutdown. Does not seem to be normal. Problem? Thanks, Tom

[scottly]

There should be no pressure in the oil tank. The way the tank was vented varied over the years; what year bike?

[scottly]

Oops!!! I missed the 70. Your bike has a spigot on the oil tank near the top that is vented to atmosphere. The hose is routed down near the swingarm pivot on the right side. The similar sized spigot on the valve cover is also vented to atmosphere, and it's hose is routed back, over the carbs, and down alongside the tank vent hose.

[PeWe]

Maybe the vent from valve cover is connected to oil tank instead of both vents are free to atmosphere?

[BenelliSEI]

Assuming there is a vent hose attached, check it. I had one that was so plugged with a “bug house” that I had to rod it out with a coat hanger!

This can also be a sign of moisture (condensation) in the oil. Last oil change?

[newday777]

Yes the vent hoses can be plugged. A great place for mud dobbers to build a nest if it sat a long time.

[toma]

Problem solved. A previous owner had connected the crankcase breather hose to the oil tank vent hose fitting..  Thanks for the leads.   tom