Assuming your starting tempertures are the same the variable is the starting angle. The carb. is already at a 12-14 deg angle when on your trailer, then at the launch ramp you are adding another 10-20 deg. this will cause a lean starting condition. Unlike a fuel injected engine requiring fuel pressure to run, your engine is carbureated and the mechanical fuel pump only keeps the fuel bowls at the proper level. Therefore the starting peroid really dosent even need the fuel pump at all until enough fuel is drawn out of the carb. & needs to be replenished. I would do two things, first try a deeper launch, even float the boat off the trailer once to see how it starts, next would be checking & adjusting to proper float levels in the primary bowls.
