My bet is that the carb idle circuit is plugged up! Do not bother with the pump. Once you fill the carb fuel bowl with gas (the squeeze bulb) it will run for more than three minutes at 1000 rpm. this is a common problem with this model mercury. the carb has to be disassembled and soaked in a good carb cleaner. the passage that you want to clean runs across the top of the main venture and come out on the back side of the throttle butterfly on the top.
