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  1. #1

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    780 SLT bogs off idle no power

    I have a 1996 Polaris SLT 780. (3cyl FUJI). To avoid questions of the sort, I do not know the ski’s history. When I got the ski, the engine wouldn’t even crank over due to the impeller contacting the housing. After my shade tree repair of that issue, here is where I stand…

    The engine will start and run fine/idle, but while in the water it bogs when throttle is pressed. Out of the water, the engine will rev up no problem. Compression in all 3 cylinders is between 115-120psi. (Don't know of a way to test the crankcase pressure to validate proper reed valve operation). I have thoroughly cleaned all 3 carburetors (they were quite nasty). Fuel pump is clean as are the fuel lines. On a couple of my test runs I could pump the primer and get the ski on plane. Out of a dozen test runs I was only able to do this twice. I started the testing with new NGK BPR8ES plugs. The PTO and center plugs look identical being relatively brownish in color, but dry. The magneto (front) spark plug is black and wet looking. Any suggestions on what to do next?


    I have thoroughly read the mikuni sbn carburetor manual posted by beerdart and have found a “happy range” for beginning the tuning of the low speed jet screw (approximately 1-1/2 to 2-1/2 turns out each). Anything beyond that range and the engine stumbles at idle. Beginning at 1-1/2 turns each on the low jet screw, I attempted to adjust them out in ¼ turn increments in order to obtain a successful half-throttle. I’ve had no luck. Bogging begins right off idle. Was hoping the wet black plug should be an indicator of something.

    Any help is greatly appreciated.

    rhislo


  2. #2
    ph2ocraft's Avatar
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    rhislo, WELCOME to the green Hulk Forums!!
    What RPM's are you able to reach WOT?
    Did you also clean the internal fuel filters of each carb?
    Does is sound like you lose a cylinder at any RPM's?
    Any spitting through the carbs?

  3. #3

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    Thanks! Looks like a lot of good info on the forums here. Hope we can figure my problem out.

    I was afraid someone would ask about the RPM's. My Multi-function-display is a Mal-functioning-display...I get no readings for anything (looks like a good bit of moisture got inside it). Thus I don't know any RPM #'s. I can't even get to WOT anyway.

    I did clean the internal fuel filters on each carb (they were actually clean to begin with).

    As far as losing a cylinder goes, I'm not certain. I did notice while stopped in the water, as waves would hit the back of the ski, I could see puffs of smoke come out of the mag cylinder carb. I thought then perhaps reed valves??? But, wouldn't a faulty reed valve affect compression???

    I saw the spark plug chart in one of the postings, but couldn't read it...got a link to a page? What is that black wet plug at the mag end telling me?

  4. #4
    ph2ocraft's Avatar
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    I think it would be beneficial for you to pull the reeds and have a peek.
    Knowing the RPM's would help determine if maybe you are hitting the rev limiter (happens at roughly 4200 RPM).
    With the MFD being defective, it MIGHT have an effect on your engine performance. I've yet to see it but many have said a new MFD fixes things right up. It certainly wouldn't be a waste of money to have it repaired or replaced either way.

  5. #5
    axgrider73's Avatar
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    Here is a plug chart that might help. Let us know what you find out.

    http://www.dansmc.com/spark_plugs/sp...s_catalog.html

  6. #6

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    I know for certain I'm not hitting the rev limiter...remember I can rev it up no problem out of the water. The revlimiter circuitry doesn't know if the craft is in the water or not. According to the schematic, the rev limiter only kicks in when there is an over temp or low fuel condition.

    As far as the MFD goes it is only an input device and has no output signals. 8 wire inputs include: +12Vdc power, -GND, Tach input, fuel sens input, eng temp input, oil level input, and 2 wires for trim indication. A bad unit that is connected could cause a problem (like grounding something that should not be grounded), but a disconnected unit should not affect engine performance. Disconnecting the MFD was one of the first things I did.

    I will check the reed valves this weekend.

    Thanks! I'll post my findings!

  7. #7

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    axgrider73...Thanks for the spark plug chart! That chart in particular is superb because it doesn't just give one example for certain conditions...there are multiple examples for the same condition. Fantastic!

    My PTO and Center plugs are showing like example 14 (the best) of course this is with limited run time.
    My Mag plug if definitely oil fouled (example 1). This tells me I'm lacking proper combustion due to either poor compression (will recheck this) or an inadequate amount of fuel. My checklist now includes: Compression check (again), reed valves, pop-off pressure, and needle seat arm adjustment(during reassembly I seem to recall the lever arm on the front carb not being in the same postion as the other two).

    Thanks for the reply! I'll post any findings.

  8. #8

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    New Findings...

    OK, for those interested....

    I pulled the reed valves to give them a look over and they looked fantastic. Then I did a compression check and the mag cylinder had 0 compression. PTO and Center still had around 125 each.

    Disassembly revealed the piston with a good size chunk broke off, the cylinder scored, and the head beat up really bad.

    In the beginning stages of getting this ski running, all cylinders had about 120psi compression. The carburetors were really dirty. I can think of a few reasons why the piston breaking may have happened (including me not going through the carbs from the beginning to eliminate a lean condition) but I need to move on and get this thing going.

    So I need some advice/direction on what to do... Do I try to find a good used cylinder and piston? OR do I get my cylinder resleeved? (And by who?) Who makes a good aftermarket piston and/or sleeve? Anyone got these parts? What are your thoughts on gaskets?

    Any help here is greatly appreciated.

    I must say using this forum got me motivated to dedicate some time to this ski!! Thanks Guys
    Last edited by rhislo; 07-16-2007 at 09:42 AM.

  9. #9
    ph2ocraft's Avatar
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    I'd check with the guys in classified and see if there's some good used stuff.
    I'm pretty sure Zigler will have something as well.
    Have you checked to see just if the carbs are plugged, or kinked fuel lines maybe??

  10. #10

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    Fuel lines are free flowing, Carbs are clear internally. Jets are clear as are the passages, ports, etc. (I've been quite the carburetor man lately as I had to rebuild my FJ1200 motorcycle carbs as well...Glad my boat is fuel injected!!!).

    Anyway...one of things I did the last time I ran the engine was leave the return line off of the Mag carburetor. I didn't get any fuel out of it while the engine was running. Shouldn't there be some fuel return? In earlier troubleshooting I had a bottle at the end of each carburetor fuel supply line and cranked the engine. There seemed to be plenty of fuel being pumped into the bottles (I did have to restrict the Rear carb fuel line for fuel to flow out the front one).

    Can't we just avoid the fuel supply problems and install a 2-5psi electric fuel pump? The lowest pop-off pressure Ive seen on any charts is 18psi, so it's not like we'd be over-pressurizing the system!?! May be a good troubleshooting tool anyway.

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