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1997 Force 120 not running at the correct top end speed

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  • 1997 Force 120 not running at the correct top end speed

    I have a 17ft sprint bass boat with a force 120 that I purchased from a. Friend that I recently rebuilt. one of the pistons cracked and scored the cylinders. I replaced pistons, rods, rings had the cylinders machined. Got the motor back together and got it out in the water. It was only pushing about 4000 rpm and 36 mph with a 24p prop. Assuming that was way over pitched I dropped down to 17p. Now I’m running right at 5000/5100. Speed dropped to 34/35. What am I missing? I was told this boat should be running in the upper 40s low 50s. The boat runs great idles smooth, sounds great, holeshot a excellent but top end is just lacking that top end speed I was assuming and told the boat should have.

  • #2
    Originally posted by AvidAngler17 View Post
    I was told this boat should be running in the upper 40s low 50s. The boat runs great idles smooth, sounds great, holeshot a excellent but top end is just lacking that top end speed I was assuming and told the boat should have.
    Sellers often exaggerate how fast a boat will go.
    Or, they just don't know.

    Don't know who made your Sprint boat, what year it is, or how heavy it is. Tried looking it up on boat spec and NADA Guides. Neither has a listing for Sprint boats.

    Is it possible the boat is heavier than it should be? Is the flotation foam waterlogged?
    Or perhaps there are limitations to the hull design.
    Does this boat have a pad?
    Medford, WI


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    • #3
      17 with a 120 should get around 40 maybe more??
      Depending on the boat size, load, and configuration. Pics of EVERYTHING?
      Weight? load? motor mounted right(not too deep in the water).
      Compression and spark test?
      You using the trim as you run?
      Have trim tabs or self levelers?



      Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
      ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

      In Scenic , Beautiful San Sebastian by the sea Florida

      88 Bayliner 21CC 2001 Mercury 225 HP EFI

      PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

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      • #4
        Find a shop with a dynamometer or a proper test prop.-------First thing you have to is check if motor can produce the power stated on the decals.-----Changing props to get the desired RPM is the wrong approach !

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        • #5
          No load other than a few rods and maybe 15 pounds of tackle. I’ve even emptied the boat with no change. I have no idea the weight. How would I get that info? I’ll get compression results and spark this weekend and get back to you. Last time I checked compression though was when I got the motor back together it was right at 120 pounds on all 4 cylinders. I do use trim yes. I usually trim down on holeshot and up until I hear the exhaust change and back down at WOT. I have suspected the motor was mounted to low. It’s a five hole plate that’s mounted 1 hole from top. I checked pad to prop and it’s well over 4.5”. Should I raise the motor?

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          • #6
            The cavitation plate should be even with the bottom of the boat.
            Watch this video.
            https://www.google.com/url?sa=t&rct=...TDESdqwRlWWD3T


            Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
            ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

            In Scenic , Beautiful San Sebastian by the sea Florida

            88 Bayliner 21CC 2001 Mercury 225 HP EFI

            PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

            Comment


            • #7
              Sorry I wasn’t able to get compression results until this weekend. I’m showing 120 112 120 112 on cylinder 1 thru 4. I haven’t been able to get a spark test done yet but I have noticed some water leaking from the gasket between power head and the mount that connects he power head to the outdrive. Could this cause my power loss?

              Comment


              • #8
                In your first post you said; you replaced pistons/rings and got the Cylinders "machined"?
                if you compression-results are that low for real (maybe re-check with a different gauge) there's a problem.
                Do a drop-down test to see if the engine runs on all cylinders / check the Carburetors for proper functioning

                Water leaking from between the power-head & mid-section means you did not properly seal the gasket.
                not causing a power loss, but should be corrected!
                "Success is walking from failure to failure with no loss of enthusiasm." - Winston Churchill

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                • #9
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                  See the cavitation plate in the pic?? It's actually 1-2" high.


                  The 97 120 was designed with lower comp on the 2 center holes.
                  You 120-112 is acceptable, low but ok.
                  Probably the gauge.

                  That water leak?? it can also be adding exhaust gasses to the cowl.
                  Try running with the cover off and see if this helps it run better.
                  I had that n one of my motors and covered the seam/seal with silicone to stop the leaks.
                  Put it on thick.



                  Buy American made Cars and Trucks!!!Towing insurance for a boat,a GREAT thing.
                  ​Tools to make my life easier: Battery tester, die grinder, temp.gun, compression tester, DVA meter.

                  In Scenic , Beautiful San Sebastian by the sea Florida

                  88 Bayliner 21CC 2001 Mercury 225 HP EFI

                  PLEASE,NO PERSONAL MESSAGES IF THE QUESTION CAN BE ASKED ON THE OPEN FORUM I WON'T ANSWER AS MY SYSTEM CAN'T REPLY TO YOUR EMAILS

                  Comment

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