I have a 1974 honda cb 750,?

Question

there is alot of built up air pressure in the oil tank,it hisses real hard right after I shut the bike off and remove the oil cap.Is this normal?

Answer 1

deffinateley the oil cap, as in on the engine?? and not the fuel cap as in on the gas tank?
if its the gas tank its due to gasoline being a volotile fuel and shaking it around causes vapour release (also occurs with temperature changes).
if its the oil cap then it will most likely be due to worn piston rings, when the piston tries to compress the air /fuel charge in the cylinder the rings seal it in, as the rings wear, it alows some of that charge to creep past, this builds up pressure inside the engine, wich when you undo the oil cap, is released.
given that its a 74 model, its highly likely that that is your problem.

Answer 2

The guy who wrote the first answer apparently does not know that the 1974 CB750 uses a dry sump system with separate oil tank. Probably the scavenging side of the oil pump has greater output than the feed side and pressurizes the oil tank, however, it would seem that the tank should be vented somehow. I owned one of those bikes quite a few years ago, but, I can't remember Take a good look at the backside of the oil tank and the cap itself for some sort of vent, if there are none it may be perfectly normal for pressure to occur?????

Answer 3

Check the crankcase breather tube coming out of the rear of the engine. It might be pinched or clogged up.