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1999 sun tracker 90hp only getting 4100 rpm

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  • 1999 sun tracker 90hp only getting 4100 rpm


    I have a suntracker with a 90hp 2 stroke. it had a 3 blade prop dont know pitch,, i was not getting over 3800 RPM. I then put a Solas 4-Blade Propeller, Pressed Rubber Hub / Aluminum, 13 dia x 15 pitch, RH on it (as this is the one my buddy has on his 22 foot landua with 2002 90hp merc) and now i get up to 4100 rpm but no higher.

    I just got the boat a few weeks ago. the motor was sputtering and popping and stalling.

    I did the following:

    cleaned carbs
    rebuilt fuel pump
    found leaking fuel line.

    The engine runs smooth , at least i think. but i have no idea why i am not getting higher than 4100
    i cant get over 16 mph.

    max rpm according to internet is 5500

    any thoughts?

    Thank in advance,

  • #2
    What size boat? How many logs? What load onboard? Speed by GPS or ?

    It would also be helpful to know the gear ratio of the lower unit. If you don't know that, provide more details about the specific engine.

    After you cleaned the carbs, did you check to see that you are getting fully open throttle plates at full throttle? Compression check ok?
    Last edited by HotTommy; July 16th, 2019, 04:48 PM.


    • #3
      Is the boat loaded down with a bunch of unnecessary "stuff"? How many people on board when you were checking your numbers?

      Also possible is that the engine is mounted too low on the transom. Only way to check that is out of the water pretty much, to see where the cavitation plate lies in relation to the transom and a line drawn across the tubes.

      I have a 24' 'toon w/Honda 90, 3 blade 13x15, turns 5000 at 26mph, so your numbers do seem low. Question will be why?


      • #4
        sorry, its a 21 foot party barge , 2 logs, myself and one other person onboard. only life jackets and spare prop under seats.

        dont know the gear ratio at all. motor serial number is 0G800001 99 tracker 90

        speed was checked using gps on phone.

        did you check to see that you are getting fully open throttle plates at full throttle?

        "did you check to see that you are getting fully open throttle plates at full throttle? "
        dont know how to check that.

        also will have to check compression this weekend when up at my camp


        • #5
          Your situation could be caused by one or more of dozens of things, but they fall into a few major categories.
          • Poor engine performance (e.g., low compression, inadequate fuel flow,, weak ignition, incorrect timing, etc.).
          • Incorrect propeller choice
          • Excessive boat drag (e.g., propeller too low in water, water in logs, etc.).
          My best guess is that your engine has a 2.33 gear ratio. If that is the case, then plugging 16 MPH at 4100 RPM into a slip calcuator yields 36% slip. That tells me your prop is trying to turn through the water much faster than your boat can travel. That implies that either the pitch is too agressive or there is much more drag than I would expect from your boat. IMHO, a 14 or 15 pitch prop should be ok, so I'd start looking at drag as the first thing to check. For example, have you seen the boat out of the water? If so are there drain plugs you can remove to check for water? ..... If you know the boat is not carrying a hidden load and the propeller is mounted at the right height, then my next suspect would be the engine. .... Your performance should be much closer to that of ahicks' boat.

          To check the throttle with the engine off, move the throttle to the full open position and look into the opening(s) of the carburetor(s). The throttle plates should be 90 degrees to the opening to allow as much air as possible to enter. If they are not, there will be an adjustment involving the throttle cable and the carb(s).


          • #6
            Tommy makes some points that I was going to make.

            Is this a new boat (to you)? Did it always go this slow make these rpms for you? I would say prop is too big but other posters have a similar pitch prop on a similar toon so while it may be wrong it's not going to be that far off. My first thought are carbs are not getting fuel all the way ie not opening up as he described. You are low on compression or more likely is that you may have a dead cylinder or two!! This is not to say that the rings are shot or something, it could just be an electrical problem like a dead coil,, or power pack, failing stator? I don't know your motor...is a tracker a Mercury made for and badged for tracker? Maybe Post this also in their forum.


            • #7
              Toubleshooting poor performance issues should always start with a complete tune-up including compression, fuel and spark tests. As pointed out, mechanical issues like incorrect throttle adjustment can cause issues."Thinking" the engine is running properly is not a valid check as many folks don't have any idea what a dropped cylinder feels or sounds like. If you don't know, leave it to the professionals to diagnose. On a pontoon dirty tubes or tubes with water in them can cut performance significantly.


              • #8
                Also need more performance information. How are you trimmed? If you go from a stop to full throttle, any cavitation? Cavitation on aggressive turns? How is the hole shot?

                What is the goal here? Just achieve the recommended WOT RPM range? Do you want to tow anyone in the water? How will the boat be used?


                • #9
                  ok guys, sorry i have not updated in a while. i had no new information so.

                  I have now gotten back from camp and here is what i found so far.

                  took boat out of water. the pontoon on the port side had water sloshing around while on the trailer. It seemed like quite a bit also. I took the boat to a woman who is a alum welder and has exp on pontoon boat. The same thing happened to me that happens to alot of people. I went to take the plastic plug off and it just broke apart in the wrench. She took a oxy torch and melted the plug plastic down into the pontoon. then drilled a hole in the back port lower pontoon and we drained a hare under 30 gallons of water from the toon. she found a 4.5 in long crack on the back of the toon on top where the deck welds to the toon. She welded the crack all up and welded a plug into the back of the toon. I am waiting for a replacement 3/8 npt plug to arrive wed to replace the plug on top of the toon.

                  I also found my yellow oil cap on the top of the motor for the 2cyl oil was not sealing at all. i have ordered another one and will have it this week also.

                  I will put the plug in and the oil cap on friday after work when i arrive at my camper for the weekend. I will also take my compression tester up with me to test the comp.

                  I will report back.

                  Thank you all,


                  • #10
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