gearbox clutch

Hey all SBO, I’m running a twin disc gearbox with a Kubota 21 hp little diesel inboard. Ive checked the linkages and tightened those, changed and topped up the fluid to the gearbox.. but still experiencing the same issue of forward gear not engaging.. so whats been happening is that after...

Yanmar SB 12 1978 Hunter 30 Need diagram of inside gearbox. Shifter arm engages forward gear just past neutral but disengages when shifter arm is all the way forward. When arm is all the way forward very slow rotation of propeller shaft. When arm is held in position just past netural very fast...