It might be a lot of things.
Where is your ignition timing set?...initial and total?
Any history on the carb?,....how long since it's been gone through? What kind of carb is it?
It's most likely carb/vacuum leak related. Have you made any adjustments to it? Raised the idle?
I would start by making sure there are no vacuum leaks....get some carb cleaner, and while it's idling, and spray around the intake gaskets and the base of the carb, as well as the front and the back of the intake...pay attention to any changes in idle speed.
Shut it off and grab a screwdriver...check the position of your idle mixture screws....just start screwing them in and count the turns (or count the 1/4 turns) until they're all the way in to establish our starting point. Then open them back up to where they were.
Check your float height and adjust that if necessary.
Disconnect your throttle cable and adjust the idle speed screw to close the throttle as much as possible and have it still idle, then open it up one full turn.....then play with your idle mixture screws. Generally, 1-2 turns out is where they should be. Adjust them in, then out, in 1/4 turn incriments until you arrive at the highest idle speed....that'll get you close. Once you get the highest idle speed with the mixture screws, readjust your idle speed with the throttle position screw to a reasonable level. It's best to do this on the water with a load on the engine.
The objective is to get the mixture set such that it will idle correctly with the throttles as closed as possible. Many people use excessive throttle angle to compensate for faults in other areas...like poor mixture or timing adjustment...and the throttle plates end up so far open that they are already into the transition circuit...which causes poor enrichment on acceleration.
Also, check your accelerator pump for working right...when you open the throttle, you should see a good shot of fuel out of the accelerator pump nozzles down the barrels. Do this test with the engine off.
Then hook your throttle cable back up and make sure the carb is closing against the screw where you set it...a misadjusted throttle cable holding the carb open into the transition circuit can cause the same problems as I mentioned above.
Try that and report back.
