.... the gas is not leaving the carb, its only cycling through the fuel pump and return lines... does it run at WOT in the water?
1. pull the carbs off, thats step one for sure....
2. completely unscrew the low speed adjustment screw and inspect the tip.... in my case the SLIGHTEST tip bend can cause major issues such as no delivering the fuel to the motor at idle
3. when you pull apart the carbs inspect the needle and seat and make sure that nothing is sticking, also check the rubber like tip to make sure its not too worn out or broken.
4. take and air compressor and be sure that efter you clean the inside out that you shoot a blast of air in the part ya just cleaned to make sure that no dirt is trapped inside the carb.
sometimes the littlest spec of dirt can cause you the BIGGEST issues.
Good luck