Chasing strange runnability problem

[Benzsavant]

I have a 1990 yamaha 25hp 2 stroke, New Plugs Newly rebuilt Carbs and new fuel pump diaphragm, New fuel filter, new primer bulb and lines
Starts right up on first pull runs smooth. Motor will run all day at 1/4 throttle with no problems. I can run smooth at 1/2 or full throttle smooth just about enough time to plane for a minute or two then will choke down to an idle speed like its starving for fuel or running on a single cylinder for about a minute before throttle comes back. At that point I can immediately throttle up again to a plane before it happens again. Happens every time the same way. Primer bulb stays nice and stiff the hole time, ive tried venting fuel tank with no change. Any input would be greatly appreciated.

 

[deepsea21]

Check your fuel tank and see if there is a fuel filter stuck on the end of the fuel pick-up line that sits down in the bottom of the tank. That is known as the "ghost filter" that no one ever thinks or even knows about. You have a fuel flow issue that develops when the engine is really trying to suck the fuel. Assuming your engine has a fuel filter under the cowling, if there's a little filter on the end of that fuel pick-up line pull it off or cut it off as it is not needed. Your primer bulb will still feel full when squeezed because it's not a leak in the system anywhere from the bulb to the carbs, it's a flow issue.