My two mercruiser engines (not scorpion) take a little longer to fire than others. I do wait to prime the fuel line before cranking the starter and never worried much about it since it ALWAYS starts. Usually one turn of the key, but may take 2-3 seconds with the starter cranking. I have heard this was more common with Mercruisers and would be interested to know why if anyone knows.
Does the boat have an anti-syphon valve? The anti-syphon valve is a check valve that holds fuel in the line between the tank and fuel pump. Just a thought. If it is worn out and leaking back it could be the reason that fuel takes a moment to prime.
is there such a thing in the gas line? if there is, that very well could be it. Very strange thing though is that i checked and both pumps are working. i could not get it to duplicate the problem either. the only difference this time is i primed it twice before i started the 45 minute session of testing. maybe the outside temp was not hot enough to cause vapor lock. Good thing is there were no leaks.