I run a ’64 RDS-26 (on salt) that has that style of box – the dealers had given up on getting it pumping properly when I bought it from them. Whilst it had a new impeller, no effort had been made to clear out the passages from the pickup tube, nor the channels that appear into the base of the pump ( both were almost completely blocked). The pump housing and area underneath was badly pitted too and looked like it’d chew up impellers over time. It all needed a lot of work before I found a NOS pump housing here. I worked hard with dental picks etc, at clearing everything from the skeg pickup, to the area under the pump, until it was clean. It really does pump properly now, but I can only assume that it was getting a small enough amount to show willing then run out as the water in the pickup passages depleted. Hope the pics help. Have also shown a ’63 Big Twin I had that has a different style of screen.
My spare box is the ’67 version on that dropped the screen & passages and relied on a pickup tube from skeg straight into the back of the pump housing. After ’71 they used a revised pump design with a single pump to powerhead intake design which possibly could be modified (am in the middle of a move with the 71 box buried in the chaos, so I am not sure the bolt pattern is the same..). Am sure there’s better experts than me here, who might be able to advise.
