I have an older OMC sterndrive, mid 1970s the one with the boot on the transom. The problem is the drive unit will not tilt up without just a little help. The motor engages, the drive unit moves approximately 3-4 inches then stops. Have tried replacing the motor, the shaft and worm gear, the clutch pack and shaft, the outer cap and the inner bushing. The drive unit only moves that little bit, then stops moving. if you put your hand under the drive and give it just a little help, it tilts. Any suggetions?