Motorcycle Repair: yamaha IT 125 1st gear jump out, shift fork, detent

Question
Hey Falcon,
I have a 1981 yamaha IT 125. It jumps out of 1st gear to netruel and some times straight to 2nd. I bought the bike not running and rebuilt it. 2nd gear was replaced as it was missing a tooth (not a dog) All gears and doggy's look good. Shift fork 3 has some ware on it. Had the cases apart a couple times to investigate but all looks good. When I have the clutch case off and shift into 1st, the drum only turns about 1/2 way into 1st and the detent doesn't seat properly to hold it into 1st. When I have the cases apart I can shift thru all 6 gears. With them together and test ride it slowly doesn't want to go all the way into 1st. Turning the drum its like the gears aren't meshing even though I crank the engine by hand. Any ideas?...ray

Answer
Hi Ray.

This problem sounds as if something is either worn too far or assembled wrong or (believe it or not) is installed backwards.

The symptoms that you are describing are identical to issues that happen when something is installed backwards.

Good luck and ride safe.
FALCON