Resolved posting my experience in the hope that it may help others
Working the list of things to check and fix on a new to me boat with an Alpha one gen one outdrive. Leg went into forward without issue and the prop locked up if turned CCW. But there was no reverse, not even a hint.
I pulled the lower leg and the shift selected turned as expected and both forward and reverse engaged nicely. Phew not a clutch/gearbox issue. But that means the issue is in the shift mechanism somewhere
I refitted the lower leg and played around with the lower shift shaft to ensure that it wasn't a simple misalignment of splines. Nope everything was where it should be with forward gear at 12:00. While playing around like this I noticed that the upper shift shaft was moving from circa 12:00 (hence good forward) but only rotating through to about ~1:50 (not quite far enough for reverse) and even with a screwdriver levering the foot around it was clearly coming to a hard stop somewhere - indicating (with the benefit of hindsight) that it wasn't just cable slop.
Pulled the out drive off and an absolute mess. The upper shift shaft bushing and seal was tired and water was clearly getting up into the area of the UJ and the shift cable area. But that wasn't the issue/solution here. Resolved regardless.
Playing around with the cable and the shift slider I got the feeling that the shift slider was unable to slide all the way forward (towards the bow of the boat). This meant the lever assembly (arm with roller) was not rotating very far - not even close to the angled surface of the bell housing. I pulled the cable inner out and was then able to play around with just the shift slider by itself. Measuring the depth and diameters of the cable end piece and comparing the shift slider confirmed that the shift slider was bottoming out on the plastic cable sheath (weird, did I have mismatched slider and cable perhaps). Some digging around down the hole of the shift slider revealed that it was full of gunk - about 15mm worth. A pick and the appropriate sized drill easily removed this gunk. The shifter slide now properly slides over the plastic sheath and brass(?) cable crimp and bottoms out on the surface of the shift cable nut. This achieves the desired movement to enable the lever assembly to move flush with the anglesd surface of the bell housing when and turn the upper shift shaft through to 3:00 (without the outsrive connected).
Posting this as it wasn't the most likely issue for poor shifting in the hope that it may help others at some point.
I guess the ingress of water into the area through the upper shift shaft seal contributed to the cause.
Working the list of things to check and fix on a new to me boat with an Alpha one gen one outdrive. Leg went into forward without issue and the prop locked up if turned CCW. But there was no reverse, not even a hint.
I pulled the lower leg and the shift selected turned as expected and both forward and reverse engaged nicely. Phew not a clutch/gearbox issue. But that means the issue is in the shift mechanism somewhere
I refitted the lower leg and played around with the lower shift shaft to ensure that it wasn't a simple misalignment of splines. Nope everything was where it should be with forward gear at 12:00. While playing around like this I noticed that the upper shift shaft was moving from circa 12:00 (hence good forward) but only rotating through to about ~1:50 (not quite far enough for reverse) and even with a screwdriver levering the foot around it was clearly coming to a hard stop somewhere - indicating (with the benefit of hindsight) that it wasn't just cable slop.
Pulled the out drive off and an absolute mess. The upper shift shaft bushing and seal was tired and water was clearly getting up into the area of the UJ and the shift cable area. But that wasn't the issue/solution here. Resolved regardless.
Playing around with the cable and the shift slider I got the feeling that the shift slider was unable to slide all the way forward (towards the bow of the boat). This meant the lever assembly (arm with roller) was not rotating very far - not even close to the angled surface of the bell housing. I pulled the cable inner out and was then able to play around with just the shift slider by itself. Measuring the depth and diameters of the cable end piece and comparing the shift slider confirmed that the shift slider was bottoming out on the plastic cable sheath (weird, did I have mismatched slider and cable perhaps). Some digging around down the hole of the shift slider revealed that it was full of gunk - about 15mm worth. A pick and the appropriate sized drill easily removed this gunk. The shifter slide now properly slides over the plastic sheath and brass(?) cable crimp and bottoms out on the surface of the shift cable nut. This achieves the desired movement to enable the lever assembly to move flush with the anglesd surface of the bell housing when and turn the upper shift shaft through to 3:00 (without the outsrive connected).
Posting this as it wasn't the most likely issue for poor shifting in the hope that it may help others at some point.
I guess the ingress of water into the area through the upper shift shaft seal contributed to the cause.
