hose from cam cover to air filter box...what purpose?

there is a 1/2 dia black hose runs from the right side cam cover top down to the left side bottom of the air box, apparnently up stream of the filter or tapped into the box but maybe outside of the filter rim?

bike was nearly up side down on a couple of hill climb crashes this weekend and found oil dripping from the bottom of my filter box

what is happening?

The hose is the crankcase breather. A little oil mist will come out of it pretty much all the time, so they route it into the airbox to keep it from going all over the place. Excess oil is harmlessly ingested into the engine. When the bike was ass-over-teakettle oil ran through the breather tube into the airbox. Voila, oil coming out of your airbox. Nothing wrong, just clean it up and, well, keep the darn thing upright.

thanks, why does the crank case need to breathe? emissions control? or other wise?

emissions control, thats it,s main reason so as to draw the fumes back through the engine and out the exhaust, burning them in the process makes for less fumes, less bad gas

thanks, why does the crank case need to breathe? emissions control? or other wise?

A crankcase breather is required to exhaust the pressure build-up from piston ring blow-by. Compression leakage that gets past the rings would then pressurise the crankcase causing oil to leak out from where it can, EG: seals, gaskets, etc. if you didn't have a breather.

I've noticed on my 510 that the inside of the airbox rubber boot where the breather tube connects is always wet with oil. Its wetter than I would like but the engine doesn't seem to use much oil?

Some say that disconnecting this breather tube and externally venting it improves engine performance but I think it would be marginal if any. :naughty:

Ditto on what Thumpin said about what the breather is for.

My 610 had the breather vented externally through the front downtube, same as the gas cap breather. It did a great job of oiling my right boot and making a mess of the bike!! I rerouted it through a catch bottle and vented the catch bottle to right above the exhaust header. What gets out of the bottle gets vaporized on the header. Looks cool when I'm at a stoplight and there's smoke coming from the front of the bike. Having the breather go into the airbox is a good thing, IMO. Not sure if venting the crankcase to the atmosphere will make any difference in performance, but I do know it'll make things messier.

emissions control, that's its main reason so as to draw the fumes back through the engine and out the exhaust, burning them in the process makes for less fumes, less bad gas

There is more to it than that. Not only is there blow-by as another poster has pointed out, but when your piston is moving downward, pressure is building up and this pressure is parasitic to engine power. By providing a place for this pressure to go, the engine can rev quicker.

Tons of thought goes into this idea in the design of inline fours.

And clearly the crankcase breather could be bigger because my breather is weeping oil past the hose and clamp from high rpm riding on my SM510R.

Hey guys,I have the same problem with what looks like massive amounts of oil settling in my airbox. One thing ive been thinking about was re-routing the hose down towards the chain with a small foam filter (rc-car intake filter) and have what builds up trickle down and lube the chain... any input?

I disconnected the hose from the airbox and sealed up the airbox inlet. Stuck a K&N filter on the end of the hose and routed it up so it won't suck water. Also installed a krank valve that creates a little negative pressure (vacuum) in the crank case which in theory, reduces power losses due to crankcase pressure. Did this on both my SM450R and TE250.

the breather hose is the one you are speaking of. If you unplug it from the air box you will notice a bit of a gain in horsepower and throttle response. basically, it is feeding dirty air to your motor. you can re-route the hose down between the bottom of the motor and the skid plate. Over time a small amount of oil will drip onto your skid plate, but it is easy to clean up. Anyway, this easy mod will give you a 1 to 2 horsepower increase. Just remember to plug the hole in the airbox with something so you don't suck any dirt in.

Anyway, this easy mod will give you a 1 to 2 horsepower increase.


Travis Epperson

Your first post and it's about HP gains from plugging a breather hose. Come on guy.

I have not read all the post, just a few that talked about oil mist. On a single cylinder engine the air volume below the piston pulses by the engines swept volume, 450 or whatever size your talking about. Thus as the piston goes down approx 1 pint of air tries to flow out that hose, then when the piston goes up it flows back in. As all the air can't make it out before the piston comes back up there is some pulseing in crank case pressure. As this air is flowing in and out of the crankcase it is important that it be routed to clean dry sorce as Husky has done. The KTM SX models route it to the back of the cases and there is an occasional problem of countershaft bearing failures on those SX models. Husky is a step ahead of KTM on that and several other engineering & design features.

I didn't say to plug the breather hose. That would make absolutely no sense. I said, disconnect the breather hose from the airbox, and plug the airbox where the breather hose was attached. If you don't think it will make any difference in how the bike runs, then don't do it.

sorry, just pushing buttons.

My 125 doesn't have that hose, so I can't try it

On my zx12r i connected the crankcase breather hose to the "clean air" system on the head.

A system of reed valves near the exhaust ports creats a vacuum, thus negative pressure to be created, within the crankcase.

I have seen a 6hp increase in hp (5k up), but that was on a 4cyl, 1200cc, 11,000rpm 190hp bike.

Many dragbikes use an electric vacuum pump to creat negative crankcase pressure, for the gains of cause.

I would be surprised if any gains are possible on a 1cyl, low reving engine but who knows???


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