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MERCURY SPORT JET 90 HP TOO SLOW

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  • MERCURY SPORT JET 90 HP TOO SLOW

    Please someone help me. I have a bayliner jet jazz 90hp sportjet which will not go above 5 mph. Things I have done so far: Rebuilt carbs, compression 120-120-122, .032" clearance between impeller and housing, impeller looks great, new ride plate gasket, engine rpm's within spec. As you can see I have invested alot of money so far and any ideas no matter how small will be greatly appreciated. Thank you


  • #2
    Re: MERCURY SPORT JET 90 HP TOO SLOW

    Have you checked the rev limiter?

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    • #3
      Re: MERCURY SPORT JET 90 HP TOO SLOW

      No I have not checked the rev limiter the rpm's seem fine 4000 or better engine sounds and runs great seems like problem is in the jet prop

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      • #4
        Re: MERCURY SPORT JET 90 HP TOO SLOW

        sorry something else I wanted to say is I am open for ANY suggestions

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        • #5
          Re: MERCURY SPORT JET 90 HP TOO SLOW

          rich, if you actually have the impeller hooked to the motor [no gear failure] the only thing that will cause you to make rpm's and not move is air in the inlet water stream of the pump....exhaust gas or air ...it is getting in there and screwing things up....

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          • #6
            Re: MERCURY SPORT JET 90 HP TOO SLOW

            about the air thing... it is my last resort, but with the ride plate gasket making a good seal i cant figure any other way of air getting in there. is .032" clearance satisfactory for impeller to housing??? is there any time of slip device on the impeller? i can turn the impeller and feel the compression strokes of the engine with no slop or play.

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            • #7
              Re: MERCURY SPORT JET 90 HP TOO SLOW

              Rich...There is a spline on the shaft that can shear if impeller gets jammed. Check the spline and the lock washer on the nut...Make sure these are ok and make sure the nut is properly torqued. I think the torque specs call 18ft.lbs...Impeller clearance should be .020-.030, .032 should be fine.just a Thought

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              • #8
                Re: MERCURY SPORT JET 90 HP TOO SLOW

                rich, one other possibility: is the reverse gate locking properly in the forward position? if it kicks back ya won't go. but my best guess is the key on the drive shaft is broken and the pieces can jam up and let you turn the motor.john

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                • #9
                  Re: MERCURY SPORT JET 90 HP TOO SLOW

                  About the spline on the impeller, I thought about that but the shaft is actually a threaded rod in which the impeller screws directly on the shaft so there is no option for slippage on that part the gate thing sounds like a possibility but the gate is staying up and out of the way of the nozzle. I am back to thinking possibly the drive plate gasket how well of a seal is needed??? Is there any other way for air/exhaust to enter the intake and cause the pump to cavitate???

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                  • #10
                    Re: MERCURY SPORT JET 90 HP TOO SLOW

                    The reverse gate needs to be up and out of the way of the nozzle to work doesent it??? It seems like the boat just doesnt want to plane off.

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                    • #11
                      Re: MERCURY SPORT JET 90 HP TOO SLOW

                      Thursday 4/18/02 put a little silicone rtv gasket maker on ride plate gasket seemed to do better for about 3 minutes in water then went back to her old self... Could the ride plate gasket be giving me all of this grief??? The gasket looks fine any suggestions anyone???

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                      • #12
                        Re: MERCURY SPORT JET 90 HP TOO SLOW

                        Yes rich, ride plate gasket/ ride plate itself will cause this...have seen many ride plates 'sneezed' off the bottom of the pump by people starting them up out of water...recheck all fasteners on the plate, caulk the gasket to eliminate any leaks between plate an hull, it has to be a PERFECT water tight seal....

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                        • #13
                          Re: MERCURY SPORT JET 90 HP TOO SLOW

                          Ok got new ride plate gasket today $57.00 not to bad but in stock got it on the boat put boat in water boat is better but not the best she will do 20 at best it just doesnt seem right for a mercury 90 with a 14' boat, could I be wrong??? If someone has their opinion on this boat I would greatly appreciate it. And thank you guys for all of the input so far it has helped alot

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                          • #14
                            Re: MERCURY SPORT JET 90 HP TOO SLOW

                            I have a regal rush 1994 , same mercury 90 hp . I just got it. I tested it in the bay and seems to go too slow. mileage metter broken. I will try the same thing put a gasket to see if it goes faster. When I hit the trottle it makes a noise like the impeller runs out of water. I will keep you posted on the results.

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                            • #15
                              Re: MERCURY SPORT JET 90 HP TOO SLOW

                              Rich...last jazz I fixed ride plate on an 'Adjusted' the trim......42 mph....with a 90 SJ...you got a ways to go....

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