25 HP Yamaha surging

[Attakapas24]

I have a 96 25 HP Yamaha. Basicly it runs fine. Starts easy, idles smooth, turns up real good. But, it surges at mid-speed and barely noticable at wot.

I have checked fuel lines, new carb kits, and idle adjustment. What can it be?

I have not taken the fuel pump apart, should I? What about timing?

I am at a lost.