These trucks have a motorcycle style constant velocity (CV) carburetor. The gas pedal operates a normal butterfly valve, but there is a vacuum operated sliding piston upstream of the butterfly valve. The piston has a needle at the bottom of it that regulates the amount of fuel allowed into the carb throat. There is a rubber diagram above the piston which uses engine vacuum to work against a spring pushing down on the piston. These rubber diagrams can develop leaks and will cause you to loose top-end performance. I would pull the carb apart, clean it, and inspect it. You may need a carb rebuild.