That is typical of the idle mixtures being set too lean. Start with them turned out 1.5 turns from lightly seated. Now, on the water, with the motor warm, try to accelerate. If she stumbles, open the top mixture screw 1/8 turn and try again. Repeat with middle carb and finally carb, testing after each adjustment. Repeat sequence starting with top carb.
After a while you should find a sweet spot with good acceleration and reasonable idle. Remember idle speed should be about 900 RPM in neutral.
Of course this requires that timing and ignition system are correct and functioning properly.
