I am having trouble staying in 3rd gear, it tends to pop out of gear if i go up above 3000 rpm then let of the gas. anyone have this problem? what should I check and adjustments?

I had that same problem prior to having my M21 rebuilt. Under load it would stay in gear, let off the gas and it would pop out. I'm no trans tech but has something to do with the syncros being worn. Most likely time for a rebuild...

Checkto make sure the linkage is adjusted correctly and its pushing the forks all the way back. If that's ok then either the faces of the gears are worn or the detents are worn.

Typical prob with muncies. How come it's always 3rd gear? Anyway like Larry said, but more than likely the only way to fix it is to replace the worn parts/gears. Just get used to holding it in till you save up the $1K to have it done or the $500 for parts to do it yourself.

The synchro "hubs" are worn (tapered). It happens more in 3rd because you're up-shifting AND down-shifting to 3rd more thanb any other. 2nd isn't too far behind.

The "fix" is to replace the synchro assemblies.

Jim

I used to add a small washer under the springs to stiffen the detents on the syncros. I like that click click kind of shift.

You may also be able to just replace the syncro cones depending on how worn the rest of the assembly is.