I'll try here and see if I get lucky with an answer...
Bike: 2003 Honda CRF450R
Recently I have encountered a problem shifting from 2nd to 3rd gear and sometimes 3-4.
The problem mainly occurs while accelerating rapidly down a straight stretch, (putting around at slow speeds it seems to be okay).
The shift lever will pull up like a normal shift, but when I release the clutch and get back on the gas I have not changed gears.
I have pulled my engine from the frame and disassembled it with the cases split.
I have checked the shift forks and they show minimal signs of wear and no visual evidence of being bent or warped.
All gears and dogs are perfect, the shift drum and ratchet device are all great.
After finding no evidence of damage I temporarily re-assembled the mainshaft and countershaft and put the cases back together to simulate shifting by hand.
Everything seems to work great, all shifts feel solid by hand.
The only thing that feels close to the problem I was encountering is when I do not put any drag on the drive sprocket it will not shift up or down occasionally because the mainshaft and countershaft are spinning at the same speed and the dogs are butting up against each other not allowing them to engage until I put resistance on the other shaft to slow it down.
I hope that makes sense, and I am hoping that somebody here is very familiar with transmissions and maybe even has encountered this problem before.
As an end note, this problem occurs constantly. It is not an occasional or intermittent problem - it happens every ride.