My Merc 4 stroke has always been very picky starting. It is fuel injected. Biggest problem is the bulb looses pressure. I always have to pump the bulb very firm when first starting, sometimes even after trolling for just an few hours.
I think I may have an air leak in the hose but also my gas tank is very low and well below the motor so this could also be an issue.
Anyway, I do have to pump the bulb very firm, I do not advance the throttle at all (proceedure for EFI) and it starts fine. Very rarely I will have to advace throttle for the flooding start proceedure. For some reason this is after I have tried to start it with a week bulb.