The first thing I would do is check the idle mixture screws for adjustment. The screws are located on the bottom plate of the carb towards the outer edges. They are hard to see and harder to get at on most marine setups. Rochester used at least two different types of screw heads ---one wqas a pointed hex head with a screw driver slot and the other with a double d head. The double d design requires a special tool for the adjustment. To adjust the warmed up engine turn each screw in until the rpm just starts to drop then slowly screw them out until you have either max vacuum on a gauge or max rpm on a tach. I usually set mine a 1/4 turn to the rich side to prevent dieing when the throttles are closed real fast as in a panic situation. On some of the very to get at screws I will solder a piece of spedo cable about 6 or 8 inch's long to each mixture screw at the time of rebuild. This will make adjustments much easier in the future. There could be other problems as well that would cause this problem such as a vacuum leak any where in the intake system. I rebuild about 30 marine Rochester and Carter carbs each season with the biggest problems being water and dried up fuel. Simply soaking the carb in carb cleaner will not usually work because the passages are small and the epa approved cleaner is nothing but pure crap for $50.00 per two and a half gallons. Each passage must be cleaned out to make sure it is totally clear. A welders tip cleaner works very well for this. Rick