The clutch linkage is improperly adjusted.

Adjust the clutch linkage. Refer to Clutch Assembly Adjustment in Clutch.

The sliding clutch gears are tight on the sides.

Adjust the clutch system. Refer to Clutch Assembly Adjustment in Clutch.

Chips are wedged between the spline of the shaft and the gears.

Overhaul the transmission.

There is a misaligned mainshaft of the countershaft.

Overhaul the transmission.