Before you go into the carburetor as has already been suggested I would check to see if there is an idle stop screw that can be adjusted. Normally that screw is located at the junction of the throttle gears visible on the port side of the engine. This screw can be used to set your low idle stop.
Second, you should have an idle mixture screw on that engine. When the engine has been warmed and is at the proper static waterline (boat in the water) turn the screw in increments of 1/8 - 1/4, turn clockwise and wait around 30 seconds after making an adjustment to evaluate the change. If the engine is/was too rich the engine will speed up, run smoother, and eventually spit lean or "sneeze" and probably quit. Back your adjustment back out between 1/4 - 1/2 turn from that point and leave it there. Your idle stop will need to be reset once you make a proper adjustment. Your mention of the fact that the engine will idle all day long indicates you probably have an internally clean carburetor.