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  1. #1

    2006 stx 12f - low rpm and top speed

    Took the ski out of hibernation today to get it ready for the season. Here is the story... Last year had to replace 2 pistons due to broken ring lands. Put 25hrs on the machine all summer and ran great without trouble... but only at 6600 rpm and 50mph. 120 total hours on ski.. Here is a strange thing I noticed... When pulling the throttle slowly, rpm and speed climbs nice and smooth to 6600/50, now here is the strange thing... there is a little more pull on the throttle like 1/4 to 1/2 inch or so but there is no increase in rpm/speed when pulling it all the way to the grip. I have checked the throttle linkage that the butterfly is opening fully. Could the TPS be out of adjustment (idle is ok). Any ideas? got a few months before riding season to tinker..


  2. #2
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    Check the thread in my signature, follow the directions and post back.

    Sean

  3. #3
    Old gas

    nope - fresh 93 all the time - drain take during storage

    Low fuel pressure

    45 psi checked on the rail before injectors - need to still pull the fuel pump and check sock, probably due for a filter change also, probably original.

    Worn spark plugs

    fresh

    Burned or leaky exhaust

    looks good...

    Damaged pump bearings, impeller, or pump housing.

    pulled pump - turns smooth and no major wear

    Compression - all cyls read about 130 dry / 150 wet on my POS harbor freight compression guage. Going to leak down test next.. at what psi? I cant think that all the cylinders are that worn out, faulty guage.
    Checked the valve gaps also, within tolerance.

  4. #4
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    Excellent. With the breakdown you need to be between 50 and 100 psi but you need to be very careful to do it, the cylinder must be held at tdc. Verify the readings on the cam position, crank position, oil temp and header temp sensor with a digital multimeter.

    Post back when you can. Can you post a picture of the plugs as well?

    Sean

  5. #5
    pic of the plugs other readings to follow.
    Click image for larger version. 

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  6. #6
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    They look a little lean (the outside 2, the inside 2 specifically), the injectors might have some debris in them. Any way you could find another set to try?

    Sean

  7. #7
    Pain is fear leaving your body.... rlovebk's Avatar
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    If these are original plugs they look like I would expect.

  8. #8
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    I am not so sure, Mr. Rod. He said the fuel pressure is 45psi not 43 (negligible difference but still higher) but on my ski when I tested the 15f inlet manifold it ran really lean and only reved up about that high. He also mentioned that they were "fresh" plugs. The tps could be bad, but they don't go out that often. I want to know what broke the ring lands last year, if the pistons were just swapped out then the problem could still be here. Top end of the rpm range is controlled by the throttle position sensor, bottom end it is map sensor controlled.

    Sean

  9. #9
    I have to think it would be something common to all the injectors as I believe all the plugs look lean. If it was 1 or 2 or even 3 plugs I would say injectors clogged.. but all of them.. something else... my gut says something is causing lack of fuel delivery. Could fuel pressure test fine on the trailer (like low flow due to lack of load) and not in the water?

  10. #10
    My name is Sean and I am addicted to STXs smokeysevin's Avatar
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    No, its a constant duty pump electrically powered. I does not depend on engine speed or vehicle speed. You could check the fuel filter, where did you check the pressure at? If it was between the tank and the filter that may be your issue, check just after the fuel rail but before the tank. Which 2 pistons failed and which cylinders did the 2 outside spark plugs come from?

    Sean

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