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  1. #1
    johnsonmtz's Avatar
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    SLT780...Help me get ready for the Starved Rock Roundup!!!

    Okay guys, this is long, but I want to lay out all the info up front.

    I'm trying to chase down a problem my '96 SLT780 has since I bought it in '04. It's an SBT motor and has about 112PSI on each cylinder. A little low on compression (imagine that....SBT), but hey she runs once the carbs clean out.

    I've checked piston wash with an LED light and I think I'm okay there. The MAG had pretty small clean spots on the ports, CEN was about thumbnail size, and PTO was black in the very middle of the piston with large clean areas on the intake ports. I gave MAG about 1/8 turn open and closed PTO by an 1/8th. I need to ride and check them again, but I think I should be fine.

    The problem that has existed for 5 years is I load up badly at idle. I seem to idle okay and if I squeeze the throttle to WOT the ski wants to take off initially but then blubbers badly. If I work the throttle it eventually clears and I take off. If I pull the plugs after idling they are wet with fuel, the rim of the plug is clean, and the ceramic electrode is white. So, this tells me I'm extremely rich. Also, when I go to start the ski I often have to hold the throttle wide open, otherwise it acts like it's flooded.

    I've rebuilt the carbs, installed new 2.0 needles and seats with silver springs (95 gram). I have about 23PSI pop-off pressure on each carb (measured with a mikuni pop-off pump) so according to specs I should be dead on. I never did check the jet sizes when I last had the carbs apart, so that's a bit of a wildcard.

    I have set the carbs according to my Clymer's manual for "models with first transition hole above throttle plate at idle":
    This should have:
    Pilot jet = 67.5
    Main Jet = 110

    Low Speed = 1 3/8

    High Speed:
    MAG = 1 1/8
    CEN = 7/8
    PTO = 1 1/4

    I thought I was good there until I double checked the manual and it lists two different carb settings for 1996. The other setting is for "models with first transition below throttle plate at idle". This set up has:

    72.5 pilot jet
    107.5 main jet

    Low Speed = 5/8

    MAG = 3/4
    CEN = 3/4
    PTO = 1

    First off, does anyone know which version of SBN should be on my ski? I seem to be close with the "above throttle plate" specs, at least for my piston wash. But low speed out 1 3/8 I seem to be way off.

    I've poured over the Mikuni SBN Performance manual (I downloaded that somwhere a long time ago ). The troubleshooting guide lists these possibilities for "Engine loads up during extended idling":

    Leaking Anti-Siphon valve - I'm not sure where this is? Is this in the carb somewhere or the tank?

    Leaking Needle valve - The last time I had the carbs apart I thought the needles were okay, but I can't remember now. I don't have any extra low speed adjusters. Could anyone bring some of these to the ride?

    Pop-off pressure too low - Nope, I'm dead on there.

    Low speed jet too large - Well, like I said before I'm not sure which jet is there. I'd rather not tear them down again just to look...but if I must.

    The final nugget I just found in the SBN manual. I need to observe the carbs at idle, but apparently a badly vibrating engine (which mine does) can cause the low speed needle to leak allowing excess fuel to drip out of the inner venturi. The more it drips, the more if vibrates......it just makes itself worse. Mikuni says the cure is to increase pop-off pressure until the dripping stops. I need to hit the ramp and watch for the dripping fuel.

    Other than that possibility, does anyone have other suggestions to get this lined out? I know being too rich is a better problem than too lean....but I'd really like to be able to jump out of the hole rather than blubber and spit until it finally goes. I could keep trying to turn in the low speed needles, but I'm not sure that is going to solve the problem.

    So, am I looking in the right direction on this problem? It's really a pain, especially when I try and tube with the ski. HELP!!!!!!!!

    Thanks,
    KJ


  2. #2
    Plrs X45's Avatar
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    This is a bandaid and not a fix but I just did this on my SLT780. I turned the low speed screws in about 1/8th and it made the "bogging" better. Not great throttle response but I am able to give it gas and I don't have to "play" with the throttle in order to get it moving.

  3. #3
    Lake Mead Bum & BTLS Mark starflight's Avatar
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    That wild card may be your issue. la90043 had kinda the same issue except his machine(96 slt780) was always lean. I don't know where his transition hole was, you would have to ask him. He swapped the jetting specs and is golden now. I think you need to confirm what jets you have.

    Another thing is the Polaris manual says you should be running the black 80 gram springs for 20-24 psi popoff, and you're running the 95 gram?

    The stutter going from idle to wot could be that the popoff is a bit too late in opening for the transition circuit. If you are at wot, then go to idle for a second, then go wot again, does she take off well? Or does it still hesitate? If it takes off good, I'd try reducing the pop pressure to around 20 psi and see how it goes.

  4. #4
    johnsonmtz's Avatar
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    Quote Originally Posted by starflight View Post
    That wild card may be your issue. la90043 had kinda the same issue except his machine(96 slt780) was always lean. I don't know where his transition hole was, you would have to ask him. He swapped the jetting specs and is golden now. I think you need to confirm what jets you have.

    Another thing is the Polaris manual says you should be running the black 80 gram springs for 20-24 psi popoff, and you're running the 95 gram?

    The stutter going from idle to wot could be that the popoff is a bit too late in opening for the transition circuit. If you are at wot, then go to idle for a second, then go wot again, does she take off well? Or does it still hesitate? If it takes off good, I'd try reducing the pop pressure to around 20 psi and see how it goes.
    Good info. Well, with a 2.0 needle and 95gm springs I get right about 23PSI. The problem is not a hesitation. When I grip the throttle off idle it blubblers terrbily. The engine doesn't skip, dip or die.....it just gurgles. You can see the extra smoke rolling when this happens. Once I work the throttle and get enough air in the engine it clears out and takes off.

    If anything, I think I may have the pop-off opening too soon. I've been reading the SBN tuning manual and it says the spring arm is suppose to be level with the carb body and no more than 1mm above. I'm wondering if I might have them bent upward too much and it's opening the needle valve too soon At this point, anything is possible I guess.

    I really don't want to tear into the carbs, but I think I might have to just so I can verify some unknowns.

  5. #5
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Step 1, Double check all varibles...translation, get in the Carbs and note everything,jet sizes,arms,etc. Easier now then at the lake when everyone else is riding.

  6. #6
    johnsonmtz's Avatar
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    Quote Originally Posted by Keddano View Post
    Step 1, Double check all varibles...translation, get in the Carbs and note everything,jet sizes,arms,etc. Easier now then at the lake when everyone else is riding.
    Party Pooper
    Yeah I know, I'm just being lazy. I'm not sure I have an extra set of carb gaskets, and if those rip in the process I'll be outta luck. I'm sure I could make some if needed though.

    I'll pull the carbs tonight if I get a chance.

    KJ

  7. #7
    SPEED KILLS, BUT YOU GET THERE QUICKER Keddano's Avatar
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    Go to Autozone, and buy some gasket materal, Go to Harbor Freight and for about 5 bucks you can buy hole punches,there are about 7 different sizes in a pack. I haven't bought a carb base gasket yet. I make alot of my gaskets

  8. #8
    johnsonmtz's Avatar
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    I can get the gasket material easily. The problem is I don't have a Harbor Freight here in town. You'd think a community with about 100,000 people would but we sure don't.

    I'm sure I can find a retailer locally that has them, but I'll have to look.

    Since my carbs are 38's I'm guessing a 1.5" hole punch would do the trick?

    Hmmmm I wonder if I could just get a small piece of 1 1/2" copper pipe and grind the edge sharp to make a hole punch? That might do the trick if I can't find the actual tool.

    Thanks for the hint.

  9. #9
    la90043's Avatar
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    do you have brand new pop off "springs" on the needle valve huh? whats it at? pop off? 24 psi? have you checked pop off?

    Im asking all this because i have new springs in mine and pop off is at 24 psi. But im gonna bend the arm up a tad to lower pop off to about 21 psi. I also have that very slight hesitation on my 780 that i can't tune with the low speed adjusters. if i screw in it stalls, if i screw out hesitation gets worse.

    by Eric starflight
    The stutter going from idle to wot could be that the popoff is a bit too late in opening for the transition circuit. If you are at wot, then go to idle for a second, then go wot again, does she take off well? Or does it still hesitate? If it takes off good, I'd try reducing the pop pressure to around 20 psi and see how it goes.

    Thats exactly what I have above what Eric says.


    Quote Originally Posted by Plrs X45 View Post
    This is a bandaid and not a fix but I just did this on my SLT780. I turned the low speed screws in about 1/8th and it made the "bogging" better. Not great throttle response but I am able to give it gas and I don't have to "play" with the throttle in order to get it moving.

  10. #10
    johnsonmtz's Avatar
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    Quote Originally Posted by la90043 View Post
    do you have brand new pop off "springs" on the needle valve huh? whats it at? pop off? 24 psi? have you checked pop off?

    Im asking all this because i have new springs in mine and pop off is at 24 psi. But im gonna bend the arm up a tad to lower pop off to about 21 psi. I also have that very slight hesitation on my 780 that i can't tune with the low speed adjusters. if i screw in it stalls, if i screw out hesitation gets worse.

    by Eric starflight
    The stutter going from idle to wot could be that the popoff is a bit too late in opening for the transition circuit. If you are at wot, then go to idle for a second, then go wot again, does she take off well? Or does it still hesitate? If it takes off good, I'd try reducing the pop pressure to around 20 psi and see how it goes.

    Thats exactly what I have above what Eric says.
    Brand new springs, brand new needles, and brand new seats. Pop-off is 23PSI. That was checked with the diaphragms off the carbs.

    Does yours blubber or does it fall flat. Blubbering, like mine, is definitely a rich condition. Falling flat is lean.

    I'm getting ready to tear into the carbs right now to determine which jets are installed and how my needle arm is positioned. I kind of suspect that my arm might be bent up a tad and is decreasing my true pop-off pressure. If that's the case, that would explain the excessive fuel at idle.

    I'll post back in a while.

    KJ

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