1982 yamaha 550 problem

She runs great on stands, and at ideal.... but when under load it spits and loses power til a few moments of giving throttle then she moves. When I ideal down to hit a turn it spits and loses power then will go again when going uphill already throttle open does fine. But hit a corner and hill and I hve to get off, give throttle n push ... ive cleaned carbs clean fuel new plugs and wires checked all lines. I am stumped any suggestions??

1982 yamaha 550 problem