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  1. #1

    SLT 780 BOGGING PROBLEM but has Fuel, Spark, Compression and runs great out of water

    Hi weveryone, I'm experiencing bogging with my 96 SLT 780.

    Stats:
    Compressions: avg mid 130's on all three cylinders
    Carbs: set to initial factory settings (nice fog at WOT)
    Good Spark
    Good RMP (6700) out of the water and with good throttle response.
    Idles well in and out of the water.

    HOWEVER,....

    As soon as it hits the water it boggs and with poor throttle response.

    It's not able to achieve 3,500 RPM and unable to "plane-out".

    A little bit of history: One minute it was running fine, cruising 55-60 MPHthen the next minute , a sudden performance drop!

    Could the fuel pump cause this sort of behavior?

    Also, the stator after the prop is severely damaged and chipped. However, this type of wear occurred over time.

    Could this cause some sort of vortex and/or cavitation issues that would in turn limit my RPM's?

    This has been agreat machine! Any suggestions as to where I might start to "trouble shoot" wouldgreatly appreciated.

    Thanks
    Bobby


  2. #2
    BlueFishCrisis's Avatar
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    I am not a pro but would like to try throwing my 2 pennies at this and see what others think:

    My stator is pretty well beat up as well with chunks missing and I am getting close to factory RPM and Speed on a 94 SLT 750. If anything, I would think anly stator issues wouldn't affect RPM as much as top speed, expecially by 3,000 RPM.

    Probably shouldn't be running at WOT out of the water - I believe 6700 RPM is the speed the limiter kicks in....... which is why you are seeing that out of the water.

    Typically when I experience bogging out of the hole is a low speed carb setting issue.... Either too lean starving for fuel, or too rich flooding the engine... But not sure how that applies to your setup.... Also, a carb rebuild may be in order, if not recently done....

    You can check the fuel pressure while running to determine if the fuel pump may be the issue...

    Not an answer, but hopefully some usable suggestions......

  3. #3
    Thanks for the input...I'll start by putting a new kit in the fuel pump...start from there. I'll keep you posted. Thanks again -B

    Quote Originally Posted by BlueFishCrisis View Post
    I am not a pro but would like to try throwing my 2 pennies at this and see what others think:

    My stator is pretty well beat up as well with chunks missing and I am getting close to factory RPM and Speed on a 94 SLT 750. If anything, I would think anly stator issues wouldn't affect RPM as much as top speed, expecially by 3,000 RPM.

    Probably shouldn't be running at WOT out of the water - I believe 6700 RPM is the speed the limiter kicks in....... which is why you are seeing that out of the water.

    Typically when I experience bogging out of the hole is a low speed carb setting issue.... Either too lean starving for fuel, or too rich flooding the engine... But not sure how that applies to your setup.... Also, a carb rebuild may be in order, if not recently done....

    You can check the fuel pressure while running to determine if the fuel pump may be the issue...

    Not an answer, but hopefully some usable suggestions......

  4. #4
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    BFC has some good input there.

    instead of replacing parts at random, why not test the fuel pump instead?

    Have the carbs ever been rebuilt? Maybe at least remove the carb covers and inspect the diaphragms for signs of wear, cracks, or holes.

    How does the elec stator test? (what are your readings according to the manual?)

    Have you tried cutting the plug wires back 1/4" yet?

    Do the spark plug boots ohm out good? (according to manual)

    What does your piston wash look like?

    Is there alot of fuel spitting out of any of the carbs with the FA removed?

  5. #5
    Thanks hmmmm...

    I checked /replaced diaphram(s). (just because I was there)

    Havent tested the stator...(yet) What are the symptoms of a bad stator?

    Did the plug wire trick. (thanks for the suggestion)

    Piston wash...I will take a look

    There is some spitting out from the carbs ...but only under load (in the water) but not on dry land..is that strange? I've tried to adjust them +/- off of manual spec but can't get any improvement and can't get them to stop spitting either... are you suggesting reed problems?

    Again, it runs great on dry land...hmmmmm

    Thanks
    Bobby



    Quote Originally Posted by xlint89 View Post
    BFC has some good input there.

    instead of replacing parts at random, why not test the fuel pump instead?

    Have the carbs ever been rebuilt? Maybe at least remove the carb covers and inspect the diaphragms for signs of wear, cracks, or holes.

    How does the elec stator test? (what are your readings according to the manual?)

    Have you tried cutting the plug wires back 1/4" yet?

    Do the spark plug boots ohm out good? (according to manual)

    What does your piston wash look like?

    Is there alot of fuel spitting out of any of the carbs with the FA removed?

  6. #6
    She likes the bike. But the ski gets her wet!!!! xlint89's Avatar
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    Did you use genuine Mikuni diaphragms?

    How long ago did you replace them? before this issue or after?

    A failing stator can have a couple different symptoms. Normally they just don't spark anymore, or once they get hot, they may start to have intermittent spark and not rev up all the way.

    Do you have a spare GOOD CDI you can try out just as a test?

    I've read that a failing MFD can cause issues sometimes too. Try disconnecting the plug behind the MFD and see what happens.

    Piston wash should tell if you're getting too much fuel, or not enough, and may point you in the right direction to remedy it.

    Slight spitting out the carb tops is normal, alot is not and indicating a bad reed. It does seem odd it does it IN the water, but not on land.

    Remember that there is no load on the engine when on land, so it's really easy for the engine to rev up. (like a car in neutral)

    One last Q. Was there any water inside the hull when the problem started, hose came disconnected, or anything like that?(possible water ingestion)

  7. #7
    john zigler's Avatar
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    it sounds as you are having a fuel related issue. check the fuel select valve. we see MANY of these fail here in my shop, even though they seem to be OK. i just replace them on all machines any more, in for service / tune ups etc.

    i would also do a spark test, while running the ski under load, to see if you are dropping a cyl, or have intermittent spark.

    i should have all parts, in stock here if you need. ( genuine mikuni carb kits, fuel pump kits, select valves, etc)

    zig

  8. #8
    You know you might be on to something there (regarding the MFD). I've failed to mention before but, the MFD goes crazy in the water never on land without the wave / rocking motion. It' displays various erroneous warnings with partial digital readouts. However, I've been told (by a technician) that the MFD is strictly an input only "indicator only device" that performs no output modifications. So, I never thought to put two and two together. I've disconnected it. Now, I'll go stick it in the water. I'll keep you posted.
    Thanks
    Bobby

  9. #9

    Thanks you might be on to something!!

    You know you might be on to something there (regarding the MFD). I've failed to mention before but, the MFD goes crazy in the water never on land without the wave / rocking motion. It' displays various erroneous warnings with partial digital readouts. However, I've been told (by a technician) that the MFD is strictly an input only "indicator only device" that performs no output modifications. So, I never thought to put two and two together. I've disconnected it. Now, I'll go stick it in the water. I'll keep you posted.

    • The diaphragms are knockoffs (froms Pat's Small Engine) but I've used thems in the past. But, replaced them after symptoms occured.
    • No water in the hull...almost never
    • No extra CDI..I'll need to go to ebay for that probably..know of any other source?
    • Will checking piston wash help at this point? as I have adjusted the carbs so often ...I'm not sure what settings caused which pattern....right?
    Thanks
    Bobby

    Quote Originally Posted by xlint89 View Post
    Did you use genuine Mikuni diaphragms?

    How long ago did you replace them? before this issue or after?

    A failing stator can have a couple different symptoms. Normally they just don't spark anymore, or once they get hot, they may start to have intermittent spark and not rev up all the way.

    Do you have a spare GOOD CDI you can try out just as a test?

    I've read that a failing MFD can cause issues sometimes too. Try disconnecting the plug behind the MFD and see what happens.

    Piston wash should tell if you're getting too much fuel, or not enough, and may point you in the right direction to remedy it.

    Slight spitting out the carb tops is normal, alot is not and indicating a bad reed. It does seem odd it does it IN the water, but not on land.

    Remember that there is no load on the engine when on land, so it's really easy for the engine to rev up. (like a car in neutral)

    One last Q. Was there any water inside the hull when the problem started, hose came disconnected, or anything like that?(possible water ingestion)

  10. #10
    I've just visited your site. Thanks for your suggestions and advise. You've got a new customer!!

    Bobby

    Quote Originally Posted by john zigler View Post
    it sounds as you are having a fuel related issue. check the fuel select valve. we see MANY of these fail here in my shop, even though they seem to be OK. i just replace them on all machines any more, in for service / tune ups etc.

    i would also do a spark test, while running the ski under load, to see if you are dropping a cyl, or have intermittent spark.

    i should have all parts, in stock here if you need. ( genuine mikuni carb kits, fuel pump kits, select valves, etc)

    zig

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