Have Boat Questions? Ask a Boat Repair Expert.
What does it do? Stumble? Just quit? Just won't go faster or more rpm?
There are a couple possibilities. If you have or can get a timing light, put it on the top cylinder and as you slowly raise the throttle speed see if the timing moves.
When you rebuilt the carbs did you use new gasket/seals? Did you remove and clean the jets or just use carb cleaner?
Don't rev in neutral above 2000 rpm.
Get a small oil can and with the air silencer off the front of the carbs spray a little fuel into the throat of the carbs one at a time. Just a little. Does the rpm go up? What does it do? Post back the results.
Try squeezing the primer bulb as you accelerate. Where did you get the primer bulb, hopefully not Walmart.
The needle valve thing is a starter, primer, it is open when cold allowing more fuel. As the motor runs it heats up and shuts off the additional fuel. Kind of a an automatic primer system. To test it just remove it, turn the key to on and watch the needle, if it moves it is probably fine.
Did you try adjusting the idle mixture?
When you accelerate does it just stumble and want to die? If so turn the idle mixture out 1/8 turn at a time, allowing 10 seconds between adjustments.
Top back of the carbs, may be under a small brass plug. #26 in the drawing. If the plug is in place carefully drill the plug and pop it out with an awl.