Hi Evan,
My guess would be you still have air in the system... You cannot move the water too fast...
Since the speed varies with RPM, the current idle speed would be the same as the old minus approximately 80 rpm. For example, if the pulley speed was 800 before at idle, the new speed would be 880 approximately. Cavitation, if it were to occur, would happen at way higher speeds.
You have air still trapped in the system somewhere...
Scott
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