Finding and fixing some minor coolant leaks, I discovered a mystery hose flopping around between the air box and radiator.

It is a small hose with a one way check valve that runs down to the crankcase.

What is it for, and where is it supposed to go?

Thanks

If it terminates into the timing chain casing, then it's probably the petrol tank vent hose that's only present on American spec. bikes. European bikes just vent the tank to the atmosphere.

Paul.

The small hose is connected to a dual fitting. The other hose is much larger and connects to the airbox; I assume it is crankcase ventilation.

I have the little collector under the 2 tank vent lines shown in all the parts breakdowns, so figured the mystery line had to be for something different. 

Thanks for the reply.

From what I can see in your avatar you have a K100RS 16v and they have an oil seperator which discharges into the can chain cover. The other larger hose goes to the crankcase.

Sorry, yes I have a 1991 K100RS 16V.

It is bankrupting me one tank at a time. Every ride burns a tank of fuel and seems to generate another issue.

First time I rode it in a spirited fashion, the final drive seal puked (just a seal). Next tank, the rear tire started to crack all the way around the tread (two new tires). Then came the fuel bubbles on the lower left of my freshly painted fuel tank (epoxy, por15 to follow). On the last tank, my bike developed a coolant leak; while repairing this I found the mystery hose. 

Rant over.

I went through the manual and found nothing about the mystery hose, but the hose from the air box that the mystery hose shares a fitting with is definitely the oil return (Nod to RicK G).  

This bike was originally from California... I accept sidecar paul's suggestion that this is some sort of fuel vapor return, and am leaning toward eliminating it. 

Thoughts?