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  1. #1

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    Need help with my SL 750 fuel issue

    Though I am new to Polaris and 2 stoke engines, I am not new to mechanical work. Here's the situation. For the past 18 months, my 17 year old son and I have been completely rebuilding the 94. The ski was involved in an on water accident which required lots of glass work, which worked out, as it allowed me to teach my son those much needed skills. Currently we have completed the build and she has been in the water.

    We have upgraded the following:

    Reed Valves - Boyensen
    RD intake grate and extended Ride Plate
    Added sponsons
    all new oil, fuel and water lines (drain and siphon)
    Triple fuel pump
    Auto cock removed
    jet drive completely rebuilt, stock
    new cosmetics, see pics
    all wiring has been inspected, all systems are functioning to spec.

    So here's my problem. I got it on the water, followed the rules I have read on here for startup, initial running and then acceleration testing. basically it will jump out of the and take off like a rocket, 7-10 seconds into running WOT, it just cuts off. engine dies, you have to crank it with the choke to get it to start up or wait around 30 seconds and she will fire. If I feather the throttle, it can maintain around 35 MPH or 4800 RPMS+/- 100 anything more, she wants to die. I have followed the adjustment criteria of no more than an 1/8th of a turn per adjustment. The ski is actually about 3/16th of a turn clockwise across all three carbs, can't figure this out, so any suggestions will be greatly appreciated.
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  2. #2
    BlueFishCrisis's Avatar
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    Welcome! Nice cosmetic work! I like the rub rails too.... Sounds like you have already done a bunch of homework throughout the build process.....

    Clarify when you say that the carbs are already 3/16 out on each? The stock specs call for a starting point of 1/2 turn out (which is counter clockwise) for the lows, and on the Highs, 1 1/4 for the MAG, 3/8 for the CEN, and 7/8 for the PTO.

    Are you 3/16 to start? Or 3/16 past these? If you are turning clockwise, you are closing the screws and reducing the amount of fuel flow.

    Did you rebuild the carbs with genuine Mikuni kits? This could be a fuel limiting issue as well.

    Is the fuel shut off valve operating properly?

    Is the fuel tank properly vented? Right after it shuts down, is there a large amount of air sucked into the tank when removing the cap?

    Did you need to do any motor repairs?

    Did you perform a leak down test to ensure the motor is properly sealed? If not, air leaks can cause a lean condition.

    What jetting is inside the carbs?

    Also - verify that your ski is a 94. The low drain on the hull would seem to indicate otherwise. Does the crankcase have cooling hoses down low under the exhaust manifold? Jetting and initial settings will be different if it is a 93.

  3. #3
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    My 94 650 had that drain on the hull

  4. #4
    BlueFishCrisis's Avatar
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    Quote Originally Posted by BryanP View Post
    My 94 650 had that drain on the hull
    What was connected to it?

  5. #5
    ripcuda's Avatar
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    Welcome to GH!

    Nice looking ski!

    Check to make sure you have the restrictor in the return fuel line. It's usually an inch or so from the end where it connects to the carb with a zip tie around the hose to remind you it's inside the hose.

    +1 for Alabama! I used to live just North of Montgomery... and my Mom just relocated to WA from Bay Minette.

    Cheers!

  6. #6

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    1. Clarify when you say that the carbs are already 3/16 out on each? Meaning the I turned them in (clockwise)about 3/16 which helped a little.

    2. Did you rebuild the carbs with genuine Mikuni kits? I purchased a Winderosa Kit from ebay.

    3. This could be a fuel limiting issue as well. Explain this, how is the fuel limited, because that is what it acts like to me.

    4. Is the fuel shut off valve operating properly? Fuel shut off valve has been cleaned and vacuum tested.

    5. Is the fuel tank properly vented? Fuel system was installed per the Shop Manual diagram, all vents were tested for operation and every fuel line length was measured and cut per the diagram

    6. Right after it shuts down, is there a large amount of air sucked into the tank when removing the cap? I have never tested this, I will check it later today.

    7. Did you need to do any motor repairs? No

    8. Did you perform a leak down test to ensure the motor is properly sealed? If not, air leaks can cause a lean condition. No, I had asked a local PWC dealership mechanic about this and he stated that he had never done a leak down on any Polaris and had never had any issues.

    9. What jetting is inside the carbs? I have to assume they are stock I never checked

    10. Also - verify that your ski is a 94. The vin plate shows that 94 are the last two numbers

    11. Does the crankcase have cooling hoses down low under the exhaust manifold? There is a hose that attaches from a fitting on the aft hull just above the syphon fittings, that runs to the left of the engine and attaches to the forward fitting on the exhaust elbow, it also "T's" at the PTO Cylinder and a small 3/8 line is attached to a fitting on the exhaust manifold.

    Hope the helps

    I think I will go try some other Carb Settings to see if the year is incorrect

  7. #7
    BlueFishCrisis's Avatar
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    Quote Originally Posted by Mrskins26 View Post
    1. Clarify when you say that the carbs are already 3/16 out on each? Meaning the I turned them in (clockwise)about 3/16 which helped a little. Turned in 3/16 from what starting point? What were they set at when you rebuilt them.

    2. Did you rebuild the carbs with genuine Mikuni kits? I purchased a Winderosa Kit from eb Members have experienced issue withe aftermarket kits as the parts are not exact to Mikuni specifications. This could be a cause for issues.

    3. This could be a fuel limiting issue as well. Explain this, how is the fuel limited, because that is what it acts like to me. Fuel may be limited by the lack of proper ventilation, which would be evidenced by air being sucked into the tank when the cap is removed. As fuel is removed, if air is not allowed back into the tank, and negative pressure will form. This can continue to grow great enough to prevent fuel from moving to the carbs. This is the intention behind question #5 and 6.

    4. Is the fuel shut off valve operating properly? Fuel shut off valve has been cleaned and vacuum tested.

    5. Is the fuel tank properly vented? Fuel system was installed per the Shop Manual diagram, all vents were tested for operation and every fuel line length was measured and cut per the diagram

    6. Right after it shuts down, is there a large amount of air sucked into the tank when removing the cap? I have never tested this, I will check it later today.

    7. Did you need to do any motor repairs? No

    8. Did you perform a leak down test to ensure the motor is properly sealed? If not, air leaks can cause a lean condition. No, I had asked a local PWC dealership mechanic about this and he stated that he had never done a leak down on any Polaris and had never had any issue It is very possible that the bottom end or crank seals are leaking. I have had issues with multiple different motors in the past. A leak down test is an easy way to determine if this is an issue. If the bottom end isn't sealed, the motor will run lean and possible result in serious damage.

    9. What jetting is inside the carbs? I have to assume they are stock I never checked I highly recommend you find out. The 92s, 93s, and 94s all had different jet specs as do the 780s. All of these carbs are interchangeable. There is no telling what someone else may have done prior to you.

    10. Also - verify that your ski is a 94. The vin plate shows that 94 are the last two numbers

    11. Does the crankcase have cooling hoses down low under the exhaust manifold? There is a hose that attaches from a fitting on the aft hull just above the syphon fittings, that runs to the left of the engine and attaches to the forward fitting on the exhaust elbow, it also "T's" at the PTO Cylinder and a small 3/8 line is attached to a fitting on the exhaust manifold.

    Hope the helps

    I think I will go try some other Carb Settings to see if the year is incorrect
    See above in ​RED Also verify you have installed the return restrictor as mentioned by ripcuda.

  8. #8

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    Quote Originally Posted by BlueFishCrisis View Post
    Quote Originally Posted by Mrskins26 View Post
    1. Clarify when you say that the carbs are already 3/16 out on each? Meaning the I turned them in (clockwise)about 3/16 which helped a little. Turned in 3/16 from what starting point? What were they set at when you rebuilt them.

    3/16 from factory settings


    2. Did you rebuild the carbs with genuine Mikuni kits? I purchased a Winderosa Kit from eb Members have experienced issue withe aftermarket kits as the parts are not exact to Mikuni specifications. This could be a cause for issues.

    I assume you are recommending that I use Mikuni only, I will make that happen.

    3. This could be a fuel limiting issue as well. Explain this, how is the fuel limited, because that is what it acts like to me. Fuel may be limited by the lack of proper ventilation, which would be evidenced by air being sucked into the tank when the cap is removed. As fuel is removed, if air is not allowed back into the tank, and negative pressure will form. This can continue to grow great enough to prevent fuel from moving to the carbs. This is the intention behind question #5 and 6.

    Got It!!

    4. Is the fuel shut off valve operating properly? Fuel shut off valve has been cleaned and vacuum tested.

    5. Is the fuel tank properly vented? Fuel system was installed per the Shop Manual diagram, all vents were tested for operation and every fuel line length was measured and cut per the diagram

    6. Right after it shuts down, is there a large amount of air sucked into the tank when removing the cap? I have never tested this, I will check it later today.

    7. Did you need to do any motor repairs? No

    8. Did you perform a leak down test to ensure the motor is properly sealed? If not, air leaks can cause a lean condition. No, I had asked a local PWC dealership mechanic about this and he stated that he had never done a leak down on any Polaris and had never had any issue It is very possible that the bottom end or crank seals are leaking. I have had issues with multiple different motors in the past. A leak down test is an easy way to determine if this is an issue. If the bottom end isn't sealed, the motor will run lean and possible result in serious damage.

    Not Sure how to do this, but I am sure it is detailed in the shop manual.

    9. What jetting is inside the carbs? I have to assume they are stock I never checked I highly recommend you find out. The 92s, 93s, and 94s all had different jet specs as do the 780s. All of these carbs are interchangeable. There is no telling what someone else may have done prior to you.

    I have to assume that a teardown is the only way to Identify the Carbs and subsequent jets, correct?


    10. Also - verify that your ski is a 94. The vin plate shows that 94 are the last two numbers

    11. Does the crankcase have cooling hoses down low under the exhaust manifold? There is a hose that attaches from a fitting on the aft hull just above the syphon fittings, that runs to the left of the engine and attaches to the forward fitting on the exhaust elbow, it also "T's" at the PTO Cylinder and a small 3/8 line is attached to a fitting on the exhaust manifold.

    Hope the helps

    I think I will go try some other Carb Settings to see if the year is incorrect
    See above in ​RED Also verify you have installed the return restrictor as mentioned by ripcuda.
    Restrictor is in the return line! I actually throw mine away by accident, found one online and installed 1 inch past the return port on the PTO Carb.

  9. #9

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    Ok, We took her to the river and ran it three times, less than 5 minutes on each run, to ensure we didn't blow it up or cause any damage.


    The tank is venting as it should, I ran it WOT until she shut down and then my son opened the filler cap and nothing was heard, so the vents are not it.


    We also used the following setting just trying to see if it ran better or differently.

    93 SL 750 carb settings - 7/8 - 1/2 - 5/8 = poor performance, engine stalled very quickly

    95 SL 750 Carb Settings - 1 - 1/2 - 3/4 = engine would operate, at WOT it would get to 5600 RPM then shut off
    I opened all three carbs 1/8th of a turn counter clock wise and the performance increased only slightly as far as duration of the engine running but no real RPM increase.

    I have not pulled the plugs because we had to idle back into the marina, "no wake" plus it was dark. Knowing that idling usually cleans the spark plugs up, so we are planning on taking all of Saturday to conduct testing. Any suggestions or recommendations would be greatly appreciated at this point, just trying to get it fully operational prior to winterizing.

  10. #10
    BlueFishCrisis's Avatar
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    Perform some plug chops on the water to see what is going on. If you have a leak in the bottom end, you will need to keep opening your screws until they are no longer effective. At that point, you will need to up the jetting. You can get to a point where enough fuel is being added to the mix to compensate for the bottom end leak, but this isn't solving the problem as the leak will likely get worse, and you will be increasing your fuel consumption.

    If it's dying at 5600 RPM, I would be very concerned. It only take a few seconds of WOT to burn a piston. I've done it.

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