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  1. #1

    Join Date
    Jul 2010

    04 STX 15F not running at peak RPM

    I have a 2004 STX 15F with 190 hours, and 180lb cylinder compression, a new pump housing with new bearings and existing prop that meets spec in clearence. I can not reach the full rpm of 7500 all I can get out of it is 6000rpm and 45mph. I changed the plugs out to see if that would help but still peaks at 6000rpm. I did have a problem with getting the ski started after I pulled a tube a while, I let it sit for a few minuits and cool down then it started with no problem. Shoud I be looking at a fuel, Ignition or electrical problem? This is what I have done to the engine prior to these problems I removed the cylinder head I thought the engine was locked up but to only find it was the pump which was locked up. I installed new gaskets on the intake, exhaust and cylinder head torqued all these to spec. I installed new pump housing with new bearings and seals. I also installed new bearings and seals on the drive shaft. I have done a fuel pressure test and have 39psi and also checked the output flow volume that looks good also. I do notice that the valve cover gets very hot the cylinder jugs, head and the exhaust manifold is cool. I have not done a leak down test only a compression test and have 180psi on all four. I purchaced this ski with the pump locked up so I can not say how it performed prior to me removing and replacing these parts.

  2. #2
    mirror's Avatar
    Join Date
    Jan 2006
    Seminole, FL
    I would suggest checking the valves for proper clearance.

  3. #3

    Join Date
    Jul 2010
    I will check the valves to see if that helps. How would the timing affect something like this? When I installed the chain to the cams I followed the service manual instructions and double checked. When I installed the chain if I was a tooth off it mada a big difference where the alignment marks lined up with the head so I am pretty sure the timing chain is set correct. The ski idels good and runs ok besides me not getting the full rpm range. When I took the pump apart I did notice a verry slight bend in one of the prop blades how would this effect the performance on the engine?

  4. #4
    Take the time to smile sirbreaksalot's Avatar
    Join Date
    Jun 2007
    Rockwall , Texas
    cam timing WILL affect performance, if it was RETARDED it could cause excess engine heat as u described

    is the prop a STOCK one ?
    does the ski reach full revs on the trailer out of the water
    , or when its sitting on trailer when on the ramp ( in the water )

  5. #5

    Join Date
    Jul 2010
    1) Cam timing looks right on
    2) The prop is stock
    3)The ski reaches full RPM out of the water

    I checked the valve clearence per service manual here are my results

    Spec. .15~.24 .22~.31
    Intake (mm) Exhaust (mm)
    #1) .18 .18 .15 .20
    #2) .20 .18 .23 .23
    #3) .20 .18 .25 .23
    #4) .23 .18 .23 .20

    Done another compression test with the engine warmed up my results: #1 200psi, #2 190psi, #3 200psi, #4 205psi
    Would this compression reduce the power enough to cause this problem?

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