David,

I would suspect the diaphram in the mixer has failed. They will get small cracks in them that will not let the engine run at full throttle. You should be able to take of the top to inspect. Need to be very careful as you can cause additional damage and replacement is the only course. If you find a small rip or tear in the rubber, I have used RTV to seal. Must let it sit for a day to dry. Wil hold up for a short time, still needs replacing.