Yamaha F9.9B power loss 1st 15 minutes

[vosljouwert]

Hello,<br />My Yamaha F9.9B has power loss during the first 15 minutes after start. The motor starts nice, runs smoothly stationary but it takes 15 minutes to be able to achieve power. When I push the power handle forward from middle position, the motor runs faster till the handle is at 45 degrees. When I push it more forward to full speed, the motor runs the same speed (I guess it produces 3 horsepower than). After about 10 a 15 minutes when I put the handle towards full speed (let's say to about 80 percent), the motor starts making a different sound and gradually it starts to make speed (most of the time running irregular). After that I can really have full speed. The motor has been checked by a yamaha technician, he changed sparks, cleaned carburator, adjusted the thermostate. The problem is still there, any suggestions?<br />Chris