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Welcome to JUST ANSWER small engines, let me try to help you !! I don't know it all, but maybe together we can get you a satisfactory answer!
Have drained the carburetor bowl to ensure that there is no water in the bowl? There is no screw adjustment on the carburetor that will help. The high speed is determined by the spring tension on the governor lever at the carburetor. Run engine manually, and see if pulling on the springs will increase the speed. If it does you can then increase the spring tension by bending the bracket it mounts to.
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Ellen
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