My 69 350 4speed wants to pop out of 3rd as soon as I shift into 3rd under speed shifting running through the gears using clutch? Any ideas why? Linkage - sincroniser?
Mike

The stop set correctly ?

Hurst FAQ:
What is the proper way to adjust my Hurst four speed shifter?

There is a 1/4 inch hole at the bottom of the Hurst mechanism that runs through all three levers. This is called the neutral alignment hole. To ensure proper adjustment, run the shifter from first into second and then back to neutral. Insert the neutral alignment pin (or a 1/4 inch drill bit) into the neutral alignment hole. If the 1-2 lever interferes with the smooth insertion of the alignment pin, remove the 1-2 linkage rod from the shifter and thread the adjuster button either in or out to eliminate the interference. Repeat this procedure with the 3-4 lever and reverse. To adjust the stop bolts, back the bolts out of the shifter frame until only a few threads remain. Push the stick firmly into third gear and hold. Screw in the stop bolt until contact is made. Release the stick and back the stop bolt out one turn and tighten the jamnut. Push the stick into fourth gear and repeat the procedure.

If the above comments have no effect, I would check detention balls in cover and or remove trans and check for slop in main drive shafts / Input shaft. This is shimable. Normally a .030 shim behind the front input bearing will fix this but there could be more wrong.

It can also be bad synchros.

My old 4-speed use to pop out of 4 gear, when it was under a load. I would hold the stick firmly to keep it from popping out... Got it rebuilt and in my case it was the synchros.

I'm not even too sure how to explain this but my M-22 used to do the same thing. The guy that eventually fixed said that I needed to go to a cam style selector. I think it's the thing on the inside of the cover where the shift forks slide in. He claimed that this can be a problem with Muncies and it would take care of the problem. Bottom line is that it shifts like butter and NEVER pops out of gear.

Do the simple stuff first and check the rod alignment and the stop bolt. Mike532 posted it above. Check the Hurst website for more info on this.