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Is the fuel line running the right way (arrow on bulb facing away from tank toward's engine). I ask this because, I've accidently put it on wrong, and the engine acted much the way your's did. I also have a friend that had similar problem's as you described, intermitently the engine would bog at high rpm and almost stall, he lost his mind replacing Fuel pumps, pick up screen's and other part's, the problem remained. One day he saw strange circles on the interior foam inside of the engine cover, "Space invader's" ? NO ! The foam sound deadening material had come loose, and everytime he powered up, the carb's would suck the hanging foam up against themselves stalling the engine out. The spring's also get weak on the fuel line connections at the tank, and motor. I had one get weak and sometimes it would hang up and not press open the fuel connection needle (fuel starved) but, only "sometimes", then it did it almost every time I went out until I found the problem. Do you have another fuel line you can try ?
mw