When i bought this 2003 RM 125 it had a transmission problem. It would pop into netural when it was in 2nd gear. I had talked to some people that had said some suzukis have this problem. But anyways i tore the bike apart and noticed that where number 20(Sixth Gear driven) Locks into number 16(2nd gear driven) were rounded off on both gears. The teeth on the gears were not rounded, just there #20 locks into #16.(see link)
So i replaced #20 and #16. After i put the bike backed together it shifted fine for about a month or so. THen i started to noticed it would pop into netural when it was in 2nd gear. So i tore into the bike again. And again, both gears were rounded were they lock into each other. I went through every gear on that tranmission shaft and they all were in perfect condition except #16 and #20. But i did notice there was a small chip in the shift drum. So this time I replaced Both the same gears, The shift drum, and both shift forks that go onto that transmission shaft. I put the bike together and its been shifting fine untill last race wich was about 3 or 4 weeks ago. And it started to pop into netural usually after a corner.
I just got the time to get to it, so im going to tear into the bike again. But im stumped this time. I dont know what it could be. Ive replaced everything that has been worn out. is there something that i am overlooking??? Ive been using belray gear saver oil and i change it after 2 or 3 rides. Srry about the long post but any help would be awsome!!!