" I am not an inboard expert s
" I am not an inboard expert so take any advice I offer with a grain of salt. However I have worked on automotive systems for a long time. I frequent this board because I have a fetish for outboards.
If you plugged the return hose with a bolt, and hooked up a pump that is drawing from the tank, and assuming that you have a good tight seal on your fuel cap then my guess is that you are forming a vacume inside the tank. Over the course of a few minutes the vacume could become greater than your new pumps ability to draw fuel. Its design is going to be such that it can push pretty hard but pulling fuel from the tank would be rather weak by comparison so only a small vacume could choke it down.
Try running it with the fuel cap removed, and if you find success then you have two options. Acquire a pump that is designed to send fuel back to the tank when the pressure is in excess of what is needed to run the engines, which is probably the feature that caused your oem pump to be expensive, or fashion some sort of filter to keep out trash and replace that bolt in your return hose with it to eliminate the vacume. Perhaps an inline fuel filter mounted such that the end that would normally be oriented toward the carburator pointing toward the tank in this case.
Should you go with a filter on the return hose you might be careful to fasten it in such a way as to prevent a siphon situation which could result in spilling fuel, and also be sure to have it positioned such the ventilation would be plentiful to keep from building up fumes.
I would personally favor the fuel pump with a return hose setup, but the latter is still an option. "
