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Honda CX500: What is the tube under the right carburetor/intake?

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What's this?:



It's on the right hand side of the engine at the rear, underneath the intake. This is the newer style of crankcase breather setup for these bikes - the later models of the CDI CX500s, at any rate. On a stock bike this should vent into the airbox, but if your bike has had the airbox removed and the air filter replaced with pod filters it's anyone's guess as to where it has been vented to. When we picked up our CX500 the previous owner was trialling running it vented to the atmosphere with no hose - NOT recommended, as any debris or liquid which falls into the hole will go directly into your engine!

In Australia, early CX500s vented the crankcase through head breathers (~1978-79) with later models having the bottle-style breather like in the above photo (~1980-1981). TI models after that had a vent in the pulser cover at the rear of the engine, leaving the above as a sealed inspection port. Other countries seem to have variations on the above years with regard to where the breathers are located.

The bottle style crankcase breather has an O-ring, which does sometimes leak. If you take it off to use the inspection aperture that it occupies be careful when you torque it back up as they can break with too much force.

The Honda part number for the breather plug ("bottle") is 12361-415-610, and the O-ring is part 91302-001-000. You can convert the older style breather setup to the one pictured above by plugging up the two old holes and adding the new setup - the breather plugs are part 90547-415-305 and the OEM hose is 95005-12460-20. Why would you do that? The system was changed due to Honda discovering that sustained running at 7,000RPM or more could cause an excessive amount of oil to be drawn out of the crankcase breather holes and into the airbox.

So, two things to remember - unless you have a breather setup elsewhere on the bike, don't seal it up and don't leave it open!