Never seen that Yamaha graphic before, but it looks very cool. Is the bike a
"mashup", or primarily stock 1973 components, if you know? Also, what rust solvent are you using?

Tom P.  - Smaller bikes had a ball bearing neutral-only detent on top of the engine held in by a screw in plug.  The shift drum star on smaller bikes is on right side of engine under the clutch cover with a spring tensioned detent lever-roller that drops into the low spots on the star when the bike is in gear (or a half depth low spot for neutral).. I like this system.

Pretty sure all 250 and larger Yamaha Enduros had a completely different system with the shift drum star on left side of engine inside the center cases...  a spring loaded plunger in the bottom of the engine, held in by a screw in plug, is what drops into the low spots in the star when the bike is in any gear or half-depth low spot for neutral.


Anyway, what might help in this case is installing the drain and detent plugs in the bottom of engine (without spring) and pouring about a pint of  50/50 mix of acetone and ATF into the transmission... and let it sit.  Best to remove clutch plates first unless replacing them.  Acetone and ATF makes a good homemade penetrating oil and might free up the plunger.   I'd also hang a heavy weight off the end of the shift lever so the star will keep pressure on the plunger which should help break it free.  Often applying steady force and letting time pass works wonders.

Great write up Mark T  

All 250 360 had the plunger but be interesting if someone's jammed a ball bearing in--unlikely but!!, & would be a 'big' one. If you pulled the blind plug thingy under the sprocket can you just tap the drum end & maybe free what's stopping it--ball, plunger etc. If you 'were' in the clutch side you' could tap it back & forth maybe one millimeter end on clearance. There's a curved tin plate sitting in by the drum pins but i think you still get 1mm movement. Strange situation & might be a dismantle job in the end. Gonna be interesting.

Yes, if I could take out that rubber plug the detent is right there. The rubber seems stiff to say the least and obviously acts as an oil seal - is it doable to remove? I had the carb off today and the brass throttle sleeve on the big spring (or whatever you call it) is completely glued in with varnish, no wonder the throttle grip won't rotate. Soaked it all day but I think I'm going to need to dry it off and hit it with some heat to expand the aluminum body - it's not budging so far

Yeah that fuel varnish on the slide is terrible but is probably like it right through the carb & jets & slide needle--everywhere. I thought our crap fuel was the worst in the world [NZ] but we're not alone it seems. Are you US or UK.
Talking the bung, you'd be very lucky to be able to reuse it as goes in quite deep.
In saying 'that' & because it goes in deep, you might be able to prize it out around the edge going side to side 180 then still be ok to refit.
There's a slight lead in on the case [thinking back 50 years].
Only thing 'is', you still won't be able to get at the plunger--close but not visible.
Only advantage is to give it an end on tap or rotate with some magic expanding thingy.
Personally i think there's something very unusual going on. But good luck with whatever you decide.

Well I had to make a decision on this bike, it was going to be too much overall cost and effort to make it roadworthy. I buttoned it back up and sold it for $500. I sold it to a guy who has fixed about 3 of these bikes up and he should be able to get it done. Thanks so much for everyone helping me! Best wishes!

Maybe ask the new owner if he could join this forum, so we can find out what happened 

We all want to know now --don't we haha!!.

Lol! I'll be sure to tell him!