At idle, the spark plugs will be a little wet. Idle rpm works good around 750-850 in gear in the water. If it's spitting, there is aleanout from lack of fuel to the engine. A carb staying open,air leak at the carbs,manifold,fuel system, or a leaf valve or gasket may be giving up. You might try to open the richen the idle jet adjustmen by 1/16-1/8 of a turn down the port side and then down the starboard side. You can also try to pump up the primer buld to see if the problem gets better/worse. Check the primer solenoid for leakage, fix it if it is leaking.