Hi Gateway, and welcome to Outdoorking. The usual cause of the symptoms you describe is lean mixture at heavy throttle. The most common way for that to happen is that the fuel filter in the fuel tank (the weighted bit on the end of the fuel pickup hose) or the gauze mesh filter in the carburetor, is dirty. Other possible causes include a split in the fuel suction hose inside the tank, or dirt in the carburetor main jet and the associated fuel passages, or an incorrectly set metering lever in the carburetor. To verify that the problem is lean mixture, try gradually applying the choke as you increase the engine speed. You should be able to keep it running at high speed with just the right amount of choke. Of course it won't idle with choke on, so this is not a cure, it is just a way to establish with certainty that your problem is lean mixture.
