B&S diamond 1/c commercial 20 hp on riding lawn tractor

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3 yr old motor & tractor, surging problem began this year after one mowing. Replaced fuel pump and carb parts replaced. Ran fine for 2 hours than started surging again. Checked fuel line and replaced fuel filter. Seems to be getting enough fuel??????? At one try ran at high speed ok but had to have choke closed ???? Need help !!!! roger at RHDER@capital.net Thanks