I have a 120 mercruiser that will not for the life of me stop sputtering as i come off idle. I have replaced all ignition components as I have just recently purchased the boat and all was of unknown age. REbuilt carb (mercarb) including new accelerator pump. Had timing supposedly checked and set. The one thing that i did notice was when I was rebuilding the carb that the check ball in the accelerator pump was hard to get out. Would some sort of blockage in the passage between the float bowl and the acc pump cause this? Or am I barking up the wrong tree? I typically fill up with 93 octane fuel on the way to the lake and sometimes I may have to refuel at a marina to finish off the day. Every time I fuel with the marina fuel(89oct.) I can hardly get on plane. I hope I have included all the pertinent information for this puzzle?!?!?
Water or gunk is at work. Have you checked the fuel/water separater? Also the screen in fuel pick up )tank), and the screen at the carb connection? Fuel vent and anti-siphon valve are likely suspects as well. Will it run better with the gas cap off?...J