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  1. #1

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    97 slt 780 problem

    hello all need some help with this one: will not throttle under load above 4500, if you get just a little air to pump will run great 6400 as soon as you let off boggs down and takes a while or air to reach max rpms. again i have tried all pop off settings 16-30 psi, all jets are correct , carbs rebuilt, tryed from 1/2 to 2 turns out on lows - closed to 1/2 on highs, with and w/o air box, new fuel pump , lines and valve, fresh fuel, cleaned filter, indexed crank, stator rebuilt by jet ski solutions , tryed different cdi , cut coil wires, comp. 120 on all cyls has sbt engine abt 2hrs on it. ski is original owner stock prop any thing i missed or ideas thanks


  2. #2

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    did you check what prop is in it sometimes when the prop is to tall it won't rev up until the ski comes unhooked

  3. #3

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    thanks for quick reply. I didnt check but dont think it has been changed orignal owner's ski.. he blew up motor and had engine replaced hasnt run right since any chance its a 96? they take very different jets Maybe heads are different, it seems like it is always acting rich, dies w choke or air box cant figure it out. If I raise pop off wont get enough, doesnt throttle at all lower it and it boggs rich, but plugs at chop read lean seems impossible..almost like at full throttle under load its still on low speed, but i can see alot of fuel blowing out of main could low and high be dumping at same time?? how f ed up does that sound on paper

  4. #4
    johnsonmtz's Avatar
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    Quote Originally Posted by al6kc View Post
    .....will not throttle under load above 4500

    have tried all pop off settings 16-30 psi, all jets are correct ,

    carbs rebuilt, tryed from 1/2 to 2 turns out on lows - closed to 1/2 on highs

    new fuel pump , lines and valve, fresh fuel, cleaned filter

    comp. 120 on all cyls has sbt engine abt 2hrs on it.
    The SBT engine in my '96SLT 780 has lasted fine since 2005, so while there's possibly a problem internally, it's not necessarily a death sentence because it's an SBT reman.

    1) Did you check timing? You can use a timing light connected to MAG cylinder and verify using timing marks on the flywheel. It should be 24* BTDC at 1500RPM. You can get this the poor man's way by first finding TDC on the MAG piston. Put a dowel rod in the spark plug hole and mark it even with the top of the hole. Now, use a micrometer to make a mark 3.54mm above that TDC mark. Rotate the coupler in reverse direction (counter clockwise looking from the back of the engine) reinsert the dowel rod. Rotate the engine clockwise until that new mark is in line with the spark plug hole. Through the inspection hole on the stator cover you should now see timing marks. Put a white pen mark on the line that is at the tip of the timing arrow. Now you can verify timing with the light.

    2) If timing is off, it's possible you have a sheared flywheel key.

    3) Have you ensured the timing mark on the stator is lined up where the case halves meet?

    4) arbitrarily turning screws on the carbs is bad news. According to the OEM Polaris Manual your carbs should be set up with the following specs:

    If the ski is a '96 use these settings:
    Low Speed Jet = #67.5
    High Speed Jet = #110

    Low Speed screws 1 3/8 turns out (all)

    High Speed Screws
    Mag = 1 turn out
    CEN = 7/8 turn out
    PTO = 1 1/4 turn out

    Popoff = 20 - 24 PSI

    If your ski is a '97 SLT780 use these settings:
    Low Jets = #70
    High Jets
    Mag = #125
    CEN = #122.5
    PTO = #130

    Low Screws = 1 1/4 (all)
    High Screws = 1/8 (all)
    Pop Off = 20 - 24 PSI

    5) Do you have fresh NGK BPR8ES plugs gapped at 0.024" to 0.028"? 2 years ago at the Midwest gathering Xlint89 was having trouble with tuning a ski and fouled his plugs. We popped his fouled plugs into my ski and it ran like a dog.....about 5000RPM max. Finally I got all the crud burned off the plugs and I was back up to normal 6350 RPM operation.

    6) Did you verify spark with an inline tester?

    7) when you said clipped coil wires, did you mean you cut the boot end of the wires back 1/4" and reinstalled the boots?

    That's all I have for now.

    KJ

  5. #5

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    hey thanks I checked timing for 03 motor .163 lines up with first small mark on 2nd set of flywheel marks, when run at 1500 its on but jumps and misses just wont run smooth stator mark is lined up with cases and carbs at 97 settings, pop off @ 20, checked spark, cut 1/4 off boot end, new plugs this things killin me

  6. #6

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    Quote Originally Posted by al6kc View Post
    hey thanks I checked timing for 03 motor .163 lines up with first small mark on 2nd set of flywheel marks, when run at 1500 its on but jumps and misses just wont run smooth stator mark is lined up with cases and carbs at 97 settings, pop off @ 20, checked spark, cut 1/4 off boot end, new plugs this things killin me

    found spark problem - plug boots and coil wires ohm test ok, but i guess under load they fail put new ones on, idle and timing marks were clear ran a little better. but back to carbs... still will not jump off line, stalls runs best with air box off has a small hesitation on take off, but will rev much better, but plugs read lean light tan almost white pop off @23 then bent needle valve arms up just above carb body , set lows to 1.5 with air box on and was ok but still wants to die unless you feather throttle lowered it to 20 and its worse

  7. #7
    johnsonmtz's Avatar
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    Maybe the low speed screws are worn or the o-rings are bad? That definitely sounds like a lean condition.

    KJ

  8. #8
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    What's the impeller part number stamped on it?

    What's the part number stamped on the CDI in the elec box?

    How does your piston wash look?



    If you bent the carb arm up too much, the needle may not be seating properly, and a low pop off pressure will only make it worse.

  9. #9

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    hey thanks again to all for your help, havent pulled pump yet, original cdi # 17120401 f8t33074 6718 78w97b, 2nd one in now is 17120114 f8t33071 5x31 78w96, think this one is from a 96 checked part # its same, piston wash looks good even a little rich dark all over ( no cleaned area or water intrusion ) i only bent arm up very little tip just touches razor blade, all jet screws look good no damage or wear o rings good, ski acts lean with air box on, dies when you hit it off line but runs much better with out it mabey a case leak would think with box off it would be worse

  10. #10
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    OK, now we're getting somewhere.....

    1. why did you replace the CDI???? The 78W97b has 24* timing. The 78W96 has 28* timing and is meant for a 2 seater that pulls less of a load. You're runnning more of a risk of burn down if you pull a tube or 3 passangers with that one installed.

    2. You need to post up some pics, especially of your piston wash. I think you're confused about how to read piston wash. All black is NOT rich!!!!! It's actaully LEAN and running the risk of piston failure.

    3. Definately need the impeller part number to determine which carb settings to use.

    4. You said you rebuilt the carbs, did you use genuine Mikuni parts?

    5. any engine mods done??? Reeds, FA, exhaust.....

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