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  1. #1

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    1996 SLT780 carb issue?

    The latest in my Polaris rebuild saga...

    Took the entire engine apart, opened the case and rebuilt everything over the winter.

    I finally got back to the lake today and got it in the water. Put a new battery in the hull and she fired right up. Without even touching the throttle at all I was cruising between 5-10 mph so I knew the idle set screw was off. The MFD is junk so I can't read rpms while out on the lake. I brought it in and close the idle point a bit, upon starting it it would bogg down and die out after a few seconds but was still running at high rpms at idle.

    I checked compression and everything was fine. Carbs are rebuilt and set at factory settings. The center and front spark plugs looked good but the PTO plug looked dry/lean. This is the same cylinder that I lean burned out last summer so I immediately panicked. I don't want to do that again so I'm looking for some advice.

    This seems like a fuel delivery issue... I pulled the hoses off the fuel pump and turned the selector valve on and fuel poured out so i think the fuel is getting to the carb.


  2. #2
    Click avatar for tech links/info, donation request K447's Avatar
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    Were genuine OEM carb rebuild kits used?

    Was the fuel selector valve replaced?

    Make sure both crankcase pulse hoses are actually connected and correctly hooked up at the carbs.

    A working RPM display will be useful in diagnosing and tuning.

  3. #3

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    Quote Originally Posted by K447 View Post
    Were genuine OEM carb rebuild kits used?

    Was the fuel selector valve replaced?

    Make sure both crankcase pulse hoses are actually connected and correctly hooked up at the carbs.

    A working RPM display will be useful in diagnosing and tuning.
    OEM kits were used

    Selector valve was not replaced, can be but appears to be working in that when I close it the fuel pump stops flowing and when I open it it begins to flow out of the supply ports.

    When you say hooked up correctly, is there a particular setup? I have all the discharge lines going to the carbs and the pulse and suction ports setup correctly. But I didn't not see if there is a special port for each of the specific cylinders.

    I will buy an induction tach but the MFD is toast and replacements are hundreds of $1 bills.

  4. #4

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    so i got each cylinder running on its own...

    i would ground 2 of the 3 cylinders and get it running on the one remaining cylinder for a few seconds. I rotated spark plugs and did this for all 3 cylinders.

    my concern was that it was not firing right up between the cylinders... could my issue be electrical in that the ignition is not firing properly? i only ask because if some cylinders fire regularly and the others aren't running at the same time that would be very difficult to tell while riding, but i can get all 3 running separately.

    also, each cylinder was set to factory carb settings but when it turned over it was still firing at an excessively high idle even after i turned the idle stop down.

  5. #5
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by bsouth19 View Post
    ... the MFD is toast ...
    Disconnect the MFD at the connector behind the display.

    Sometimes a a dead MFD can cause weird engine running problems.

  6. #6
    Click avatar for tech links/info, donation request K447's Avatar
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    Quote Originally Posted by bsouth19 View Post
    so i got each cylinder running on its own...

    i would ground 2 of the 3 cylinders and get it running on the one remaining cylinder for a few seconds. I rotated spark plugs and did this for all 3 cylinders.

    my concern was that it was not firing right up between the cylinders... could my issue be electrical in that the ignition is not firing properly? i only ask because if some cylinders fire regularly and the others aren't running at the same time that would be very difficult to tell while riding, but i can get all 3 running separately.

    also, each cylinder was set to factory carb settings but when it turned over it was still firing at an excessively high idle even after i turned the idle stop down.
    You may several problems happening at the same time.

    When the engine is running semi-normally a weak cylinder will feel cooler than the other cylinders since it is not firing and producing the same amount of heat.

    The engine was just rebuilt, correct? You may need to do a leak down test to confirm the engine is in fact airtight.

    If there are air leaks into the crankcase the engine will run lean and idle can be higher than expected. It is also impossible to properly set the carburetors and get the engine tuned with air leaks.

    Air leaks can happen at the crankcase seam, at the shaft seals, the intake manifold or even the reed gaskets (going from memory).

  7. #7

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    Quote Originally Posted by K447 View Post
    Quote Originally Posted by bsouth19 View Post
    ... the MFD is toast ...
    Disconnect the MFD at the connector behind the display.

    Sometimes a a dead MFD can cause weird engine running problems.
    MFD is disconnected

  8. #8

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    Quote Originally Posted by K447 View Post
    Quote Originally Posted by bsouth19 View Post
    so i got each cylinder running on its own...

    i would ground 2 of the 3 cylinders and get it running on the one remaining cylinder for a few seconds. I rotated spark plugs and did this for all 3 cylinders.

    my concern was that it was not firing right up between the cylinders... could my issue be electrical in that the ignition is not firing properly? i only ask because if some cylinders fire regularly and the others aren't running at the same time that would be very difficult to tell while riding, but i can get all 3 running separately.

    also, each cylinder was set to factory carb settings but when it turned over it was still firing at an excessively high idle even after i turned the idle stop down.
    You may several problems happening at the same time.

    When the engine is running semi-normally a weak cylinder will feel cooler than the other cylinders since it is not firing and producing the same amount of heat.

    The engine was just rebuilt, correct? You may need to do a leak down test to confirm the engine is in fact airtight.

    If there are air leaks into the crankcase the engine will run lean and idle can be higher than expected. It is also impossible to properly set the carburetors and get the engine tuned with air leaks.

    Air leaks can happen at the crankcase seam, at the shaft seals, the intake manifold or even the reed gaskets (going from memory).
    I did a leakdown after the rebuild and while it did not hold a vacuum it did hold positive pressure. i could not find any bit of a vacuum leak and figured the vacuum leak had to be with my temporary block off for the test.

    with positive pressure on it, i soaked the case in soapy water and got nothing close to bubbles. i occasionally pressurized it too much and would shoot my plugs a good 12-15 feet up in the air.

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