Did you leave the backfire suppressors in or were they breaking up like the ones in this motor. They say you don’t need them and you should take them out as the pieces can end up in the crankcase. The one with the arrow pointing at almost made it. I believe that its the backfire that breaks them up. I’ve run 2 Johnson’s with this power head and find the mixture touchy as well so its easy to get to the lean side and – BANG!!!
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.