This last year I blew the powerhead (after new fuel pumps were installed by me, the gasket wasn't installed straight and blocked the vacuum hole, this caused a lean condition that damaged the #2 piston ,rings and cylinder wall,new powerhead by a yamaha mechanic,$5700.00 later) and after a new powerhead was replaced the motor kept dieing when at idle. The yamaha mechanic said it was because the motor wasn't broken in yet? So after putting up with the poor idle all summer ,I checked the carbs and the yamaha mechanic had set two of the tree carb floats to 22mm instead of 16 mm from the gasket. I put carbs back on and motor ran perfect at idle ,finally! This last Saturday I took the boat out and with the smooth conditions tried a high speed run to 5100 rpms.As soon as I throttled down to 1500 rpms the motored started running very rough and stalled at idle,It wouldn't start for about 10 minutes ,then started and ran very rough and black smoke was coming out the exhaust. The flow scan showed 3.0 gph at Idle when normal at 700 rpms is 0.7gph.The compression was normal and plugs were wet and black. Did a float in the carb stick open or what , Thank you. P.S. new carb kits and fuel filters last month.
