I've posted a pick here, I think the rear cylinder of the starboard engine is getting water in it, does this look like a piston which has been steam cleaned around the edges?
I've checked spark and that is good, i've checked fuel delivery, all good. It will start on dry land in idle with no choke no problem, in water its harder to start and needs full throttle to keep running, I was thinking i was getting water in the cylinder when I back off the power? sometimes you can have full throttle and you will get nothing then bang all the power comes at once?
Sorry my knowledge of these engines is limited but with guidence i'm an ok diy mech.
Some more info, this engine has also been killing spark plugs, the colour of them is ok they seem to just die after being used in this engine, also it died on me once and seemed like it was running on one cylinder,
I think your 95 speedster came with 657X motors I have 2 motors in a 95 speedster that was stolen and recovered before it was totally stripped in 1995 so the engine basicly were only used for a couple of months, and have been sitting all this time. If you wind up needing an engine or two let me know I doubt you could find any engines that year in this good of shape.