1972 wr250 stuck in 5th gear

I think I know the answer to this but just posting in the hope there's a simple fix.
I had the engine rebuilt on my wr250 and after some confusion on my part with the carb I have it running nicely.

There is another issue now that has become apparent, I clicked the shifter up 4 and now it won't click back down, the gear lever moves down but doesn't engage on anything to take it back down through the gears.
Any ideas before I split the case and look inside?

If this is a new rebuild, sometimes there is not enough lube on the all the trans parts to let them move freely. Just a shot in the dark, did you try to push the bike forwards and backwards with it in gear to see if it will free it up?

Marty

Hi Marty.
Yes fresh rebuild and yes tried pushing it back and forth, kicking it over and trying to change gear.
I had a modern-ish gearbox oil in it that's a bit thinner than it should be, would that make any difference?

take the clutch cover off and take a hard look at the shifter parts, probably stupid simple what's going on

What Premo says above and if can't be resolved then cases will need to be split.

This is the first place to look after you split the cases.

Thanks for the response. I can't see anything untoward with the clutch cover off.

Looks like it's going get a bit more intrusive.

Yes, removing the clutch cover only exposes the slot for the shift lever to engage, not the actual selector mechanism.
Having just been through this exact scenario on a 72 250WR, I split the cases and followed the instructions in the workshop manual shown above and can now select all gears up and down, but haven't had it running yet.

Update: now running and shifting like it should, only to discover that it has the wrong piston fitted, a post 74 from my research, which does not reach the top of the cylinder, around 5mm short, causing very low compression.