Hi all, I'm having some trouble getting my CT1 to downshift back to first gear. If the bike is cold I seem to be able to start in neutral  and downshift into first gear with no issues. Once I start riding it I'm able to upshift and downshift from all gears except first but I hit neutral and the gearshift lever feels hung up and won't go any further. If I shut the bike off I can usually pop it back into 2nd then into first. When the problem occurs it feels wedged with no up and down looseness like it's locked between gears. The clutch is worn out and is slipping but just I ordered the 5 new friction plates. Lower end oil is new but may be synthetic 4T. I've resigned myself to be prepared to spit the cases to resolve the issue but hoping there is some way to have an idea of what is causing this. Anyone experience this issue in the past?

Check that the shift lever isn't hitting something.

Check that the neutral detent spring and ball is present under the plug behind the neutral switch. 

Make sure the shift drum retainer and shim are present...  located under the three bolt cover just forward of the sprocket.

Other than that, hopefully it's something in the linkage you'll see when you pull the clutch cover.
 

Your gearbox oil could be contributing to your problems , both the shifting and clutch slipping .
You were a bit vague about the oil itself , so not sure whether it's been changed since you
owned the bike . Car type oils have a number of friction reducing additives .
Good for a car application , but not so for a wet clutch in a motorcycle .
You need to use a motorcycle specific oil , probably 10 - 40 grade .
Too heavy an oil grade , can cause some clutch drag , making the shift down to first gear
hard .
Try fresh oil , many recommend Yamalube 4T , and you may solve both your problems .

Remove the clutch & also remove the gearchange shaft or just hold the claw arm up off the pins & move it through the gears by turning the drum with a 'good' fitting screwdriver. Neutral detent will grab it hard in neautral with ball & spring on the left top. But roller arm will make it hard to turn through too from gear to gear. But go through gears which will require slight moving of the main shaft--clutch shaft--to line up the dogs. Other think 'is'--are the bits in place holding the drum in place under the three screw plastic cover--or alloy on some early models--above the sprocket. If all that checks out then drop the arm onto the pins & go through it again. Not normal that's for sure so just closely watch it operating, & for what's hanging it up. Sprocket nut done up. Might've popped the circlip on first gear big wheel but not really those symptoms.

When I mentioned the spring and ball...  emphasis on the ball in the correct place...  I've seen someone put the spring in and then the ball on top under the plug.  Spring would jam up the drum.

Upon running through the gears  I think This lock between first and neutral may be the action on the shifting turnstiles. I noticed the fork can become lodged under the first pin. The plate also shows some signs of wear. Not sure what to replace to correct the issue. 

Oh, that's one i've had before on an RX125 but didn't come to mind this time. The stop screw is eccentric so should be able to adjust it within its one turn, or half a turn really, to centralize the arm over the drum pin when in gear. Probably try 3rd gear to clear neutral half movement between first & second but doesn't matter as once set its the same for all, just avoid 1st & top gear. On second thoughts it might be easier in neutral as less travel to pickup the pin to go up or down a gear so easier to adjust & watch a small movement. So now i've confused it all--good luck.
ps by memory on the RX125 i had i rounded off the pointy end on the arm or at least thought about doing it, but in hindsight it must be like that for a reason. Was about 40 years ago.

I would agree with the above. Your shift fork should not travel far enough to get under the pins. The eccentric stop bolt/ shift fork assy. has a problem. Not adjusted correctly, loose, missing altogether, or the "window" in the fork worn out.
alnarv 

This is great news. Thanks guys! I assume I'll need to finish removing the clutch basket and the adjustment point for the eccentric is below the basket? I'll check the service manual section to see what I can learn there. This problem only seems to occur when downshifting to first. No other gears seem to have this issue. Perhaps there is just enough wear on the first or second gear pin to allow the claw to slip below the pin. I do see some obvious friction wear on the plate that holds the pins right around the first gear pin. Looks as though that plate is still available. 

Just a quick update on the issue getting first gear. The claw was well out of adjustment. Looks like the adjuster nut has become loose. A couple turns of the adjuster to center the claw again and Bob’s your uncle. It’s too loud and too dark to test it on the road but it seems to shift perfectly. Broke some major rules and mixed up the order the parts go back together but I sorted it out. One more thing to check off the list.