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It sounds like you are missing the clean out plugs. Here is the part numbers for the cleanout plugs.
F0R-67690-01-00 It will take 2 to fill both holes. They retail for about $224 each.
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NO, you would loose close to all thrust if the clean out plugs are not installed. If you are getting the sensation of a slipping clutch with the plugs installed, I suggest checking the clearance between the impeller blades and housing. If it is excessive, the pump will cavitate.
If the clearance is ok, then look for any areas on the intake that would let air into the pump system.