I had the same problem and found out that it was a problem with the carb, it was getting to much air and by using the choke it increases the gas. Usually caused by a problem in the main jet. Can be rebuilt or adujested if you have one thats adjustable. I rebuilt mine a couple of times and finally got a new one and a adjustable jet from Bendix for about 125.00.
Good luck with your boat. If you have any mechanical ability these engines are easy to fix yourself.
John