Since I haven’t found any information at all regarding my problem with the gearbox breather hose, I’ll post the solution in english also, for all of you guys who don’t speak swedish.
The problem began with oil leaking from the drive shaft on the bike, a 2009 KTM 530 exc Sixdays. Since this is a quite common problem I simply replaced the seal and bushings, which of course did not fix the problem. The oil was leaking kind of sporadically, more often after short rides than after long rides, and it didn’t leak much..
So, this summer I took the motor apart, replacing the crank shaft seal on the right side (which is usually leaking), replaced the oil pump seals, housings and shaft and replaced the piston rings. I also put new seals everywhere it was leaking. Yesterday when I went for a test ride after rebuilding the motor, it still leaked.. 🙁
When I stopped I could hear a kind of gurgling sound from the filler cap on the gear box, and as I opened it it sounded like opening a soda can. There was a lot of preassure inside, which should be vented out through the breather hose! When I tried to apply preassure to the breather it was sealed. No matter how hard I tried to blow into the hose, I couldn’t push any air through. After some research I opened the cluch cover on the motor and removed the cluth and kick starter. If you put your bike on the side like this you won’t even have to drain the motor oil:
The holes in the gear and the bushing are supposed to be aligned, to let the air in and out of the breather hose. For some odd reason my bushing was misaligned and no air could escape the gear box, which is why it started building preassure inside as the motor got warm and the air and oil trapped inside expanded.
The solution for this is pressing the bushing out of the gear, realigning them correctly and – using some kind of loctite – pressing it back togeather again.
The fix worked, did a quick ride of about 10km today and not a single droplet of oil was underneath the motor when I got back. First time since I got the bike that that’s happened!