Can someone explain the function of the tube circuit that runs from the top of the cylinder through the airbox, and down to the bottom of the bike, ending in a capped rubber tube?

I recently took out my airbox in desire of replacing it with a large K&N filter. Ideally I want to know what to do with the remaining tube that runs from the top of the cylinder to the airbox. Small breather air filter?

You may wish to first run the hose up as high as you can under the tank (this provides an up slope for oil vapor to condense and drain back into the engine) and follow that upslope with a run way way down low to a drip point that will fall free to the pavement.  

Any kind of little in-line mesh fuel filter element mounted at the end of the tube would be fine for this use since all you are doing is keeping the bugs and dirt out of it (keeping the passage way nice and open while allowing vapors and liquids to pass freely).  

For small out of the way end points for puke tubes and carb vent tubes I have used these little things as they are small, cheap and easy to hide.


Your breather tube is going to occasionally puke some oily liquid stuff that you don't want landing on your engine -- generally when you are running WFO on the interstate, etc.    It will also smoke a bit as your engine gets older and the amount of blow by increases.

Every year or so unplug the little filter and blow it clean with a reverse blast of carb cleaner -- it will last a long long time with only very occasional cleaning.

Thanks, guys. I did my hard krome last weel and I'm just finishing up the big K&N soon. I'll be on here again with my results.