Re: 115 merc.
I had the same problem when I first got my boat. I ended up having to do the carbs, replaced the small fuel lines, and clean the fuel filter in the fuel pump.
Check your fuel bulb to see if it stays hard after it starts. If not it could be a bad fuel pump or the bulb could have a air leak. I am not an expert but I have learned this information from the other very informative persons on this thread. I never even rode on a boat (other than a military ship) until a few weeks ago.
