Yamaha 115 carbureted. One of the cabs was gummed up so took in to service both carbs. Mechanic also replaced in line filter. He lake tested it and said it was OK. I took it out and at low and medium speeds it's fine, but open it up to about 5000rpm and it starts surging. It will drop down to about 3000rpm, then up to about 4000rpm then back up to full power, then will lose power again. Mechanic replaced fuel pump. Still the same. I am bewildered. It was not doing this before I took it in to have the carbs serviced.