Re: 1965 90 hp prop shaft movement
The normal movement is primarily due to the clutch dog in F gear. The clutch dog surrounds the drive shaft and is spring loaded. In F gear it's teeth mesh with the teeth on the rear of the F gear which is constantly turning due to it and R gear being in continuous contact with the pinion gear mounded on the drive shaft.
When the rotation of the prop shaft is driven in the CW direction (by the drive shaft) the clutch dog locks into the rear of the F gear much like a saw blade. That's where you get thrust. However, when the prop is in the overrun condition, which is the case when you are on plane and cut the throttle, the prop continues to turn in the water faster than the drive shaft is turning and making the overrun condition. This relationship is like going backwards on a saw blade like happens when you are drawing the saw back towards you for the next cut.
You said prop shaft, not prop. That is important! The only thing in there that could go whacko that I can think of is the spring that holds the clutch dog in place or the pin going through it that locks it to the shaft, or the spring that pushes the cam follower into the cam. But if either of those problems existed there are a few things that would/could be noticeable: Shifting would be unreliable, prop shaft would pop out of gear, and where would these parts go and how is it that they broke....a far fetched situation.
If however, the prop is loose on the prop shaft, which makes a lot more sense, then you apparently have the wrong thrust washer for the prop hub used, or you have it installed incorrectly. For a quick clue to being right, when you get the prop installed to where you think it ought to be, on the tail end of the prop shaft, aft of the properly tightened nut will be about 1 to 1 1/2 threads left behind the nylon insert.
Mark
