Crankcase venting

5twins

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A big part of the problem on the early models (besides too much oil) was that Yamaha made the breather outlets too big. Through the '70s, they tried several "fixes". First one was to cap one outlet off completely and only use the other one. Next try was going back to the 2 outlets but with rubber reducer plugs fitted into them. Finally, they redesigned the breather housing, changing it to a single outlet with a very small hole in it. You can retrofit this late style single pipe breather housing to your early bike if you like.

1r5RXWZ.jpg
 

barncat

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Well, this is a subject I'd never given much thought to- I just assumed Yamaha engineers knew what they were doing when venting to outside air on the early motors, and getting some chain lube in the process. They do mist a significant amount of oil.

These brake check valves must be rubber diaphragms or reed valves? I have to confess i don't see how they are supposed to restrict oil loss if they still communicate with outside air...
 

5twins

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They only let air out and not back in, and this creates a slight vacuum in the case. I guess that's what keeps the excess misting down. I started out using a small K&N filter on the end of my open breather hose. I had removed the rubber restriction plugs following a tip from another web site. That turned out to be a mistake. The filter was always very wet with oil. I put the plugs back in and switched to the power brake check valves. I've never seen any oil come out of them. Their outlets are, at the very most, just a little damp with oil.
 

barncat

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Noted.

From what i can find these valves are just spring loaded plungers. I'm pretty surprised if they're holding up to millions of cycles over time. They'd ordinarily see orders of magnitude less as brake parts.
 
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