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Discussion Starter · #1 ·
I’m a new owner of a 1981 Honda cm400 and in the middle of cleaning up electrical stuff and cleaning the bike I noticed the previous owner had changed the air box to two cone filters and I’m assuming that there was a hose coming out of the top of the engine that went to air box but there is nothing there now and my question is does this effect anything and should it be filtered too, covered, or just leave it alone it will be fine?




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That's a vent hose alright, you can put a filter on it if you really want to but be sure to clean it at regular intervals if you do. Many vintage Hondas just run this hose back under the seat along the frame.
 

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Welcome to the forum. More pics would be welcome. And you'll need a proper Factory Service Manual for reference. PM sent.
 

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Discussion Starter · #5 ·
I'll be interested to hear how well it runs with the pods, they can be problematic with the CV carbs.
Yeah I’ve heard some bad things about them which is why I didn’t do it my shadow but I don’t have the stock air box so I’ll just have to see myself I only got to ride it around once before I started taking the wiring harness apart seemed to bog down a lot but not sure if that’s the clutch cables fault or not because it’s got electrical tape over the end to keep it on the lever so I had to push it back by hand for the clutch to disengage but I got a new one on the way and hopefully will have the mess of a wiring harness figured out by next weekend so I can get a good ride out of it to see how she does



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Discussion Starter · #7 ·
That's a vent hose alright, you can put a filter on it if you really want to but be sure to clean it at regular intervals if you do. Many vintage Hondas just run this hose back under the seat along the frame.
Thank you thought there might be something else to do with it to make sure no junk or water goes in


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Yeah I’ve heard some bad things about them which is why I didn’t do it my shadow but I don’t have the stock air box so I’ll just have to see myself I only got to ride it around once before I started taking the wiring harness apart seemed to bog down a lot but not sure if that’s the clutch cables fault or not because it’s got electrical tape over the end to keep it on the lever so I had to push it back by hand for the clutch to disengage but I got a new one on the way and hopefully will have the mess of a wiring harness figured out by next weekend so I can get a good ride out of it to see how she does
you'll get all the help you need here for the 400... but don't forget this one below

 

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There will be more water vapor than oil coming out of the hose. If you have a mist oil coming out of it you have a problem.

Best to just route it to the back of the engine so it can just drop on the ground or fit a catch can to collect what little bit will come out of it.

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Some water vapour yes,but no more than the weather here. Have a look inside the hose, what do you see inside it after say 100 miles? or at its end when directed to the rear of the bike, but yes point taken, as least the tin mudguard will be less corroded, or more? due to the water vapour?
 

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You can put a check valve in the hose and air/moisture can't get in. That will also create a small amount of vacuum in the crankcase when the engine is running, vacuum in the crankcase helps the rings seal better and will dry up small leaks on the outside of the motor. Some folks claim a small power boost, having a check valve installed, due to better ring sealing. All I know is I could feel the difference when I installed one on My Norton. A power brake booster check valve from the auto parts store is a cheap way to try out the idea. Note: this works best on singles and 360° twins, 180° twins don't have near as much volume of air being pumped in and out of the crankcase, a check valve wouldn't make much difference.
 

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I'll vote for a catch can, just remember to drain it every oil change. Keeps the bike cleaner not having oil/water vapor sprayed on it.
 

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I just ran a new hose on mine for the same reason. I zip tied it up under the tank about 5 inches away from the attachment point. It is elevated so I am hoping most of the oil spray will run back into the motor. The hose will exit down under the seat into a small filter. I may run it down where the fuel overflow lines are but it is hidden pretty well know.
 
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