Can someone help me with a yz85 transmission

This is a second post looking for specific answers to fix my son's near-new yz85 (15hrs). It shifts fine at low rpm's, but wont shift from 3rd to 4th (or 2nd to 3rd) under full throttle. I'm inspecting the shifter mech. and it looks okay but only goes into the higher gears if i roll the trans axle, same thing coming down, which seems normal but i know nothing about the transmission. Ive changed four -stroke cranks but never had to mess with the tranny.

My next step is to split the cases but not sure what to look for. My question is why would it shift smoothly except when wide open?

Did he Ever come down really hard on the shifter?

No wrecks. But he has tried to shift it pretty hard like on holeshots.

You likely wont find anything really wrong - other than it doesn't like to shift under load (a good thing IMHO).

 

I've had some hard shifting bikes and our fixes start with new shift forks and shafts, the yz 85's tear up 3rd gear so inspect it for any pitting.

Why would you think it's "a good thing" that the bike wont shift under-load? So you're saying split the cases and look at the forks.

alot of bikes dont shift at full power because the torq is holding the gear dogs together.  hold it full throttle and blip the clutch while shifting at the same time

That's what we are trying and it's staying in 3rd gear. Like on a holeshot: wide-open never letting off..... 2nd, clutch, 3rd, clutch, ......but it wont go into 4th.

All MX bikes use a "constant mesh" type tranny. Technically you cant shift "under full load". There has to be a slight "unloading" of the gears to shift. You also don't need to use the clutch. The ideal acceleration shift is WOT, slight quick rollback of the throttle, shift, back to WO throttle.  It is advisable to use the clutch, as clutchless shifting will ultimately add  wear to the gear dog pins and slots.

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