Motorcycle Repair: 1963 c105 honda trail 55

Question
It's stuck in 3rd gear. It was running fine and then it sat in the garage for a bit and now seems to be stuck in 3rd gear

Answer
Julian, what is "for a bit" ?  Over long periods of time the clutch plates can "stick" together, so that it won't disengage when you are riding it..

With the bike on the centerstand try rocking the back wheel back and forth while you work the shifter lever up and down. We'll assume that the bike hasn't crashed or fallen over and bent the shift lever/spindle end that would lock it in gear.

Worst case scenario is that you would have to remove the clutch cover and peer into the shift mechanism to see if something is binding, bent or jammed with the shift selector parts and/or the shift drum itself.

http://images.cmsnl.com/img/partslists/honda-c105-worldwide-except-usa-gear-shif

You should be able to rotate the shift drum with a screwdriver on the shift drum screw inside or you can look underneath the countershaft cover and use the 10mm sized bolt head to turn the drum. If the drum has seized, then you will have to tear it all down to see if a shift fork is bent or something else has caused this issue.

Bill Silver