I can offer a few things for you to try/think about. But first, know that starters arent designed to be continously turned over (I really had to struggle w/my .12 when it was new too) and you can burn them out (you can feel the starter heating up). Short bursts is better.

As for starting a difficult engine (Try these steps in order when you first go out to start it):
1) loosen the glow plug about 1/2 turn.
2) Put your finger over the carb and tur over the engine (n bursts ) til you see fuel going into the carb thru the fuel line (if you have see-thru lines) OR till fuel comes out around the loosened glow plug.
3) Once you KNOW the carb is getting fuel, pinch the fuel line going into the carb (this will require a 2nd person or a hemostat etc) and turn over til the engine fires up and then let go of the fuel line. (The engine should stay running if you keep blipping it - assuming the needle is somewhere close and there isnt something else going on).
4) If it does die, take out the glow plug and spin the fuel out of the engine (careful not to spray yourself).... take the fuel line off the carb and while pinching it, pick you boat up and point the nose to the sky to let any excess fuel in the pipe/header flow out (this can build up and cause and engine to be flooded).

These are the steps I was shown when I was fighting w/my new engine.

I'm still a little confused/having a hard time visualizing what you mean when you say that when the engine does fire up and run, it splutters. Are you blipping the throttle or not once running?

For what it is worth, the starting will get easier and easier as the engine gets broken in. I really had a hard time to get mine to fire and had to turn it over a lot (but I learned that once it fired up even for a few seconds it would soon fire up and run - seemed to need the heat). It wasnt til probably 15 tanks before it got easy to start (now it fire "cold" in about 1 minute - once its been run for the day, it starts in @10-15 seconds).... so dont let it beat you!