YZ250 1st Gear Problem

Hey guys, I recently rebuilt a 97 yz250 top to bottom. I rode it for some hours last year and it ran great. Now this summer i brought it out and the first thing i notice is when im in first gear the bike jolts as if its missing a couple teeth. But all other gears work fine and everything shifts great.

So I tore it back apart to find no apparent failures in the tranny.

I have all the diagrams and have gone over several times to make sure nothing is missing and its all put together properly.

Some insight - this bike previously had tranny problems and i replaced a couple gears and a shift drum.

So my questions is even if i did put the tranny together wrong why did it work for sometime and then start having this problem? To me it would make more sense if something where to come loose to cause this problem.

Also now that i have the case split when i spin the tranny by hand at some points the tranny binds. I've heard ppl talk about bent shafts causing this problem.

Give me your thoughts I don't want to put this back together without finding out for sure what the problem is.

Check the engagement dogs on first gear. See if any of them are rounded off or broken/chipped.

I checked and all gears are in good shape, even if it was slightly worn i couldn't see it causing a problem that extreme. As i was taking it apart i was expecting a total mess for the first gears.

I'm not talking about the teeth on the gears. Look on the back side of the gear where it engages the gear next to it on the same shaft. There are protrusions refered to as "dogs" that fit into the gear directly adjacent to it on the same shaft. If these are worn out then it can jump in and out of gear giving you that lurching of the bike.

Any update, 2nd gear is doing a similar thing on my 125

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