I've been working on rebuilding a 1981 Honda CB900C. Managed to get the bike mostly complete, but during it's first real test drive, once the bike got hot, it started popping out of gear. At first I thought it was false neutral between some of the gears, but it became really difficult to put into first at a dead stop without it popping right back out again. I could get it moving by pressing the shift lever as I was letting the clutch out, and it would sometimes stay in gear, but would sometimes pop back into neutral. When shifting into second, it would seem like it shifted into third or fourth instead, and the whole transmission seemed really screwy.
Figuring it was something I did, I pulled the clutch back out, along with the external shift mechanism and triple checked that everything went back correctly. After this, I could click through all the gears like normal while the bike was off, so I put everything back and tested it again. Same issue. When cold, the bike would shift normally, if a bit short for the speed o was going, but I chocked that up to the bike itself, as I had never ridden it before, save the one time. But when the bike got hot, the same problem came back. Popping out of gear and just general shenanigans.
Fast forward, I've got the motor pulled and the crankcases split, and I noticed on the counter shaft, one of the gears was on backwards. It's one of the gears engaged by the shift forks, specifically the countershaft 4th gear. Upon closer inspection, it appears that the dogs on that gear and the one it mates to, have a bit of wear indicating that it was slipping out. In the orientation it was in, the dogs could only engage about 1.5mm deep into the countershaft 1st gear.
Here's the thing: there's a 25% chance that I pulled the gear off and put it on backwards myself, although I remember specifically trying to keep the shaft in one piece until I had inspected it, and I don't think that gear ever came off. Another detail is that the crankcase had been split and sealed before, so the transmission has likely been removed before, and could potentially have been put back incorrectly.
My question, at the risk of being a dumb one, is this: would it even be possible to make a mistake like this and still end up with a somewhat drivable bike? Would the backwards gear explain the behavior during the test drives? There are no visible signs of wear on any of the transmission parts, other than the aforementioned that seems minimal. Everything is within spec and in remarkably good condition for the year. What else could cause the symptoms I've described?
Any help you guys (or gals) could offer would be appreciated. This bike has been a massive undertaking and being so close to the finish, I'd love to see it out right.