it may be worth checking the cable adjustment, but its a long shot.
if you pull the lever gator off, there is a 6mm hole, that lines up with another. you may have to access from under the cover. on later levers, there is a pre installed lock, horseshoe shaped, that just pushes in when aligned.
under the vehicle, left side of the gearbox, you just release the clips on the ball joints, allow the cables to move if they need to, then squeeze back together.
if that doesnt work, it sounds like the coffin shaped teeth inside the gearbox are worn. does it crunch that gear if you chenge as fast as you can?
thats a gearbox strip if so.
tony, May 2012
