Ok here are some questions maybe dumb though. Is there slack in the throttle cable at the carb at closed throttle at the handlebar?? Is the choke completely off: have you actually looked for an open choke plate in all the carbs?? And perhaps the dumbest question: Is the main curb idle screw between the second and third carbs adjusted properly? If the carbs were syncronized, can we can assume that pilot jets are adjusted properly? Hope these questions help. Let us know what you find.