83 Honda CM 250 C -Crankcase Breather - Honda Rebel Forum
 
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post #1 of 8 Old 09-29-2018, 02:32 AM Thread Starter
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83 Honda CM 250 C -Crankcase Breather

Hey all, my bike came without a carb or air filter box. So I strapped up a new set of carbs and added a pod filter. I read somewhere that you can't just add a small filter to the crankcase breather port.

Has anyone figured out a way to get around this problem? ie make their own set up?
I'm not trying to search for a new box with the whole emissions system set up.
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post #2 of 8 Old 09-29-2018, 02:34 AM Thread Starter
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woops

Just for reference, I've got Mikui VM32-33 Carbs, with KN pod filter.
It's on a 1983 Honda CM250C Custom.
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post #3 of 8 Old 09-29-2018, 08:01 AM
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You could make you own "box" to run the breather tube into. If you rigged a drain tube with a duckbill, it would self drain and not allow dirt into the system. Something like this: https://www.ebay.com/itm/Aquarium-Mu...4AAOxyf1dTJ~Nf

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post #4 of 8 Old 09-30-2018, 07:11 PM
 
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Hey Nathan,

I'm used to working on cars, so I might stand corrected when it comes to bike stuff.

However, from what I know, the point of having a tube that goes from the crank-case to the air-box is for the suction created by the air intake to also create a vacuum in the crank-case, which helps with piston ring sealing and draws out oil vapors/emulsions created by the rotation of the crankshaft. On the stock Rebel there's an air-oil separator that condenses the oil vapors and sends them down the puke tube, allowing only clean air into the air-box.

On old-school performance cars it is relatively common to delete the PCV system (the automotive equivalent to this), but there are some trade-offs:

By removing the system, the piston rings won't seal quite as well and the engine will experience more "blow-by" leading to a slight loss of power (in cars this is usually largely compensated by the power gained from a more free-flowing intake). Note that on a bike, since the intake faces away from the direction of airflow around the vehicle, the stock air-box may or may not be better at channeling air into the engine than an open intake, I have no idea.

Long-term, the increased "blow-by" may wear your piston rings a little bit more, and retention of crank-case vapors may incrementally affect crank journal wear.

There should be limited harm in sticking a small filter straight on the crank-case, provided it is free flowing enough to prevent buildup of positive pressure, but prevents dirt and particulates from entering the engine. A step up from that would be to run the tube to some other vacuum source (I've seen things like exhaust bungs, but they do have draw-backs).

To summarize, by removing the air-box and crank-case vacuum, you're trading a bit of longevity for better looks, lighter weight and perhaps more power... (provided you put good enough filters to keep dirt out of your engine). Whether the small amount of increased wear will cause an engine failure before any other issue does is anyone's guess.

Sorry for the long post.

-Blue

1985 Rebel 250 (in blue of course).
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post #5 of 8 Old 10-01-2018, 07:41 AM
 
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Sometime back there was a thread posting pics of "custom shop built bobbers" and they used what appeared to be a small pod filter on about a 1" bit of breather tube, pointed straight up so that the oil spray would mostly drain back down (but keep an eye on them getting filled/clogged)

I think it was something like these or one of the "also like...":
https://www.amazon.com/BQLZR-Intake-.../dp/B00RUQNYIM
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2009 Honda Rebel 250
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HRF Answer #1 You should take the MSF Rider Course
HRF Answer #2 You need to clean your carburetor
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post #6 of 8 Old 10-01-2018, 10:52 AM
 
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Quote:
Originally Posted by kryton View Post
Sometime back there was a thread posting pics of "custom shop built bobbers" and they used what appeared to be a small pod filter on about a 1" bit of breather tube, pointed straight up so that the oil spray would mostly drain back down (but keep an eye on them getting filled/clogged)

I think it was something like these or one of the "also like...":
https://www.amazon.com/BQLZR-Intake-.../dp/B00RUQNYIM
This is a bad application for this type of filter, since it really has no "clean" side. To the extent that it sucks air from the outside in, it works like a normal air filter, but water/oil vapor and sludge buildup on the inside will produce invisible fouling after a short time. The stock system does not attempt to filter the oil mist beyond separatiing the worst of the liquid to the puke tube. It simply reburns the blowby gases using filtered air and fresh fuel.

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post #7 of 8 Old 10-01-2018, 01:27 PM
 
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@kryton: @Duckster is right about this, air moves out from the crank-case, and these little filters have paper elements that will soak up the oil and flow less and less as time goes on. You'd generally want to go for a filter with a foam element for a crank-case breather. You might get away with a paper element one if you keep the air-oil separator in between the case and the filter. Having a tube sticking straight up is a good idea, as it acts as a sort of oil baffle.

1985 Rebel 250 (in blue of course).
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post #8 of 8 Old 10-02-2018, 06:00 AM
 
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Oh I never claimed a filter straight on the breather tube without the OEM seperator & air box was the "best" answer, but the OP was looking for an expedient 'good enough' alternative to open air venting the crankcase since they already had a pod air filter setup.


Quote:
I'm not trying to search for a new box with the whole emissions system set up.
Yes, you can just slap a filter on the crankcase breather, provided you are aware of the issues/trade-offs (listed above).

2009 Honda Rebel 250
"The bravest thing for me to do is admit when I am wrong" - unknown
HRF Answer #1 You should take the MSF Rider Course
HRF Answer #2 You need to clean your carburetor
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