I have a '75 MX250B (says the 509 VIN).
Been a fun little dirt bike, up until today I had a turn over when my rear spilled out on loose sand.
I have the double peg shifter (1 lever, front pushes down to downshift, rear pushes down to upshift.. pretty cool).

Bike used to shift beautifully with no issues, but after the minor turn over it gets stuck in first/neutral; but I think I've forced it to stay in 2nd.
The front of the shifter is rubbing against the crankcase, this has not happened before.
I can engage the clutch and get the wheels to move no problem- just more resistance than neutral.

I'm fairly handy mechanically, but have never had the cases split; what needs to be done here to properly diagnose?
Do we have tutorials or expanded diagrams on a bike this old?
Any similar models I can compare to?

You'll likely have to pull the clutch cover... sounds like shaft got pushed in and linkage came out of time. Fairly common.

I figured as much, is it as easy as you make it?
Any exploded diagrams I can reference?

Including some pictures of the linkage from the exterior.

It's been discussed on here many times. Here's a fairly recent thread.

yamaha-enduros.com/index.php/forums/gene...shifter-seal?start=0

Also any of the 74-76 DT250 or DT400 manuals in the tech library here would be a good general reference. Very similar to your engine.

Ah, definitely looks like that shaft got pushed in.
Will be pulling that clutch cover after all then; hopefully something easy.

Attaching some photos if you notice anything else abnormal

Make sure you have #4 and #5 intact. See attached parts diagram

www.partzilla.com/catalog/yamaha/motorcy...975/mx250b/shifter-2

Just built a center stand today and plan on cracking open the right side clutch cover tomorrow.

Is it best to drain the oil first? Or do so while the clutch cover is off? I've seen guides that show one plug containing a detent spring that generally should not be lost.. Any guidance there?

Also realizing this is the twin shock subforum and not the MX. Sorry mods!

Think I've got it.
First picture shows that the shift lever plate was totally off-centered, and not even on both sides.
Taking the clip off the shifter peg side allowed me to push the shaft over towards the brake side (right) which freed up the shift lever plate. It auto-centered, which was a huge relief.

Then reached back with a finger and adjusted the gear that it comes in contact with (red arrow) until it lined up.
Shift lever plate is no even, and seems like we're good to go.

I'm assuming there's no way that I can accidentally set the 'neutral' wrong, and once it lines up it will always line up correctly (I tested 1-N-2, but it didn't want to go further than that).

Think I may need to tighten up the clutch cable next, as it only seems to 'engage' at the end of the lever pull.
Thoughts on repainting the magnesium? I'm reading that it's very porous; wondering if sandblasting and baking a few cycles would be the key.

Don´t put the magnesium in the oven, you will end up with puff pastry.
Don´t ask, how i know...i tried to powder coat
Better a 2 component semi gloss paint

Was considering a wire wheel and some aircraft remover to get the old paint off. Some light sandpaper and cleaner before laying on high temp black gloss.
But it being magnesium makes me a bit nervous, the battle scars give it character, too..