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Sounds to me like your throttle cable is sticking or has a spot that it gets caught. Recheck your cable from the pedal to the carb. An assistant would be very helpful to push on the pedal while you check.
I just rebuild the carb on my old chevy truck and the throttle was doing the same thing you described. I sprayed a lubricant in both ends where the cable goes in and out of the housing and it made a huge difference.Hope this helps!!