I just bought a boat with a 60hp mercury. Currently you have to put it in gear and ease the throttle to get it to go. If you give it throttle too fast the engine will die. The boat was only going 3000rpm at full throttle. I took the cover off and held my hand over each carb to make sure it was firing on all cylinders. I did one carb at a time and noticed change in rpms when i cover each one. After that the boat was able to go up to 4000 rpms. I got to the fishing spot and turned it off. When I left it would only go to 3500 rpms. Any idea what the problem could be