Re: 1955 Evinrude 15 HP fuel pump upgrade???
not sure on this particular one, so i will answer in a very general fashion.
most 2 stroke outboard fuel pumps are powered by crank case pressure pulses. When the engine is running in a (two stroke engines case) the piston on the up stroke creates a vacuum (or low pressure) and draws fuel through the carb and reeds and into the crank case. On the down stroke the piston pressurizes the crank case closing (or allowing to close) the reeds, then the pressurized fuel air mix gets PUSHED into the cylinders.
This gives you one strong pulse per cylinder for every revolution of the engine. If you so choose to, or have the need to, you can drill and tap the case insert a threaded x barbed connector and use that line to power a diaphragm operated fuel pump. I haven't done this on an outboard before, but that said, I would not hesitate to do it if I had a fuel pump at hand and the need for one.
Things to consider 1st, where is the pulse that pressurizes the fuel tank currently coming from (my 1st guess is there is already a hole in case, and a reed (check valve) in place allowing you to pressurize the tank) can you use this hole to power your fuel pump?
If you must drill and tap your case, ideally you would take the engine down so that you could be absolutely assured that there would be no chips escaping from the drilling tapping operation. In reality this is generally considered to be too much trouble so the alternative is to drill very slowly using grease as a lube to catch the chips, if done with the right angle cut on the drill bit itself, the right pressure and speed you can get two clean ribbons of aluminum to follow the flutes of your drill and NOT get any in the case. Then comes time to tap it, at this point you do the same thing, use grease to trap the chips from the tapping operation, and remove and clean your tap regularly. I would feel amiss if i didn't point out that you should try this out and practice a wee bit 1st prior to attacking your dearly beloved (and expensive to replace) engine. This drilling operation is common practice in the Radio Control communities with both gas and nitro two strokes (though most times it is to provide a port to allow pressure for a smoke system on a RC plane though sometimes for remote or relocated fuel pump).
Remember to very carefully look at where you intend to drill, and what is BEHIND it before you drill. Make sure your drill bit is newly sharp and of correct size for the tap your going to use, make sure the tap is clean and don't break it off in the case. Also try to pic a location that won't interfere with ANY thing else. Oh and lost but not least by any means..... get the fitting 1st so that you know for a fact that you can get one you like before purchasing the tap and drilling the hole.
and make sure there is enough meat on the case (thickness) in the location your choose to get 3 or 4 threads cut in, less can work but 3 full threads is the minimum that I feel comfortable recommending
hope this was helpful in some small way.
Sean