Crankcase breather hose mod | GTAMotorcycle.com

Crankcase breather hose mod

Matt Rain

Matt Rain

Well-known member
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So I've been chasing this intermittent issue where my bike will sometimes lose power, bog down and refuse to idle when fully warmed up. I can't replicate it on command - some days it'll happen, some days it's fine. I can still ride it, but it's clearly down on power and it constantly needs throttle to prevent it from dying.

What I notice is that the clean side of my airbox collects quite a bit of oil. I rode in to work this morning and it bogged down. I opened the airbox and used a couple rags to remove the oil from the airbox. There wasn't a lot, but the entire airbox is covered in a fine oily film.

Anyhoo, after cleaning the airbox and reinstalling the (brand new) air filter, I started the bike and it magically settled into a perfect idle.

What the hell?

What I'm thinking is that my crankcase is spitting out too much oil and an excessive amount of warm oil vapors get into the intake when the engine is fully up to operating temp.

Thinking of disconnecting the crankcase breather hose from the airbox, plugging the hole in the airbox and going for a ride to see if it helps.

Bad idea?
 
How is your oil level? If you overfill it this will happen
 
It's right where it should be, between both marks in the sight glass. I have a center stand, so it's easy to check.
 
Lots of Harley folks do it - usually when they replace/upgrade their air cleaner. Some kits have little pleated filters on the end of the hose to catch the oil mist but still allow ventilation. I've seen photos online where they attribute carbon build up on the pistons and combustion chambers to the breather oil being sucked back into the intake.
 
One other thought - how is your oil consumption over time? If you're consuming oil, it could be something like worn or stuck piston rings letting too much blow-by gases through. If you're not consuming oil, then you could try only adding oil to the lower mark (less than spec'd).
 
FLSTC said:
One other thought - how is your oil consumption over time? If you're consuming oil, it could be something like worn or stuck piston rings letting too much blow-by gases through.
Click to expand...

I go through quite a bit when commuting (I do short loops around town during the week - ~20kms a day.) Need to top up the oil (150ml or so) at every other fill-up, except when going on longer rides. Compression is excellent even when cold, but I suppose I should do a leakdown test.

FLSTC said:
If you're not consuming oil, then you could try only adding oil to the lower mark (less than spec'd).
Click to expand...

Good idea, will try that.
 
You can vent the crankcase breather to the ground, the only reason it vents to your airbox is for EPA regs so that the oil gets sucked back into the combustion chamber to be burned, rather than dropping all over the ground.

You can use the K&N breather filters just fine too, just make sure to clean it every once in a while.
 
you can vent to a catch can or filter or to the ground, if venting direct to ground make sure its not going to get onto your rear brake/rotor and try to keep it out from directly under the rear tire, seal off the a/c fitting/hole and your done. this will also keep your plugs from fouling less.
 
I googled a lot of similar mods and I see that a bunch of guys install the filter on top of a T-fitting and they run tubing towards the back of the bike and then straight down where it drains to the ground. Doesn't the open end defeat the purpose of the filter?
 
I dont think you would need that, it might actually make the breather less efficient because when the pressure in the crankcase is high and its venting, its creating more vacuum at the filter end causing air to get sucked in. Im not sure though. I would just use either the filter or the hose to the ground, thats what i've always done and ive never seen it cause any problems.
 
I got the filter but have yet to figure out where to put it, so I just ran a clear hose towards the back and out the side. Was expecting to see some type of oily mist through the hose, but I got nothing even after a spirited ride.

My intermittent issue persists, and now my bike is even more polluting than it was.

Might just be a sticky float in the end.
 
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