Not many of these to see these days but I have one that is misbehaving. Practicing a few weeks ago I lost the ability to shift gears. Didn't get banged. Lever resisting up a down movement on the spring but no selection. Thought the problem would be the locknut on the eccentric cam coming loose. But not the problem. Have stripped the mechanism in the primary case. Nothing seized and no broken springs. Moving through the range of adjustment allows a setting that will allow going up the box or down but not both. All gears select easily using the square 8mm peg. Timing marks lining up correctly. No noises from gearbox so I think it's the mechanism in the primary case. Any tricks anyone knows to solve this selection problem please?