Not sure I can generalize but in my case there appeared to be too much endplay in the mainshaft. The dogs on the gears that engage for 3rd gear (I think it is the middle two??) were not engaging far enough or rather they were engaging but then slipping out. I had exactly the same symptoms as you describe, falling out of gear once I let off the throttle. Under load it seemed OK. I put a 0.065" shim between the inner case and the mainshaft and it solved the problem. As was pointed out by a few folks on my original thread, it is essential to also check primary chain alignment if you shim the right side of the mainshaft. Interestingly, when I checked my primary chain alignment, it wasn't very far off at all with the 0.065" shim on it. So the primary alignment must have been quite a ways off inboard in the original configuration but I never checked so will never know for sure!
The amount of endplay in the mainshaft will also depend on whether you are using a gasket on the inner gearbox cover, how thick it is etc.
Regarding your question about whether there was significant slop in these gearboxes when they were new, I can't answer that. Maybe someone else on here can though! My guess is that there was probably a fair bit of variation in the tolerances in these gearboxes from bike to bike straight from the factory and wear over the years has pushed some of them out of alignment enough to cause problems. If you look at the photos I posted, I could only detect minimal wear on the shifter forks and gears themselves which could also cause the symptoms you describe. When I pulled the gearbox apart, it seemed to me like the components were in pretty good shape and most of the respondents to my post on this forum seemed to agree. This got me thinking about other potential causes.
Last edited by Tobin Peever; 05/16/12 6:50 pm.