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  1. #1

    94 SL 750 runs but need to play with choke to keep idling

    Hey guys, there are a lot of posts out there with people who have carb issues including similar problems like mine, but I feel I've covered most of my is my problem. I bought this ski, rode it, blew the PTO piston, found greenhulk and started buying parts. New piston kit, bored jug .20 over, perforned a leak test as well as indexed the crank. Compression right now after having out a couple times is MAG 120, CEN 125, PTO 125. I've installed a new triple outlet pump, replaced all fuel lines including the ones in the gas tank, I have the restricter in the return line, I removed and drained/cleaned the tank, removed, cleaned and rebuilt the carbs with genuine mikuni parts, have original size jets installed for the 94 SL 750 (90 main jet and 75 low speed jet). Low speed screws set at 1/2 turn out. Needle and seat is a 2.0 which is stock with black 80 gram spring installed..did not test pop off since I was told on here with that set up, there is no need. When I start the motor, I pull the choke, hit the starter and it will run for a bit then die if I don't pull the choke even when I get it warmed up. If I pull the choke and release it, it clears up, then slowly peters out. If I play with the choke and the throttle I can get it up on plane, but I can tell it's still running rough and if I open it up, it's almost like it's either starving or too rich. I have the high speed screws about a 1/2 turn past stock for now since I don't want to run it too lean and if I can get it to run good at idle/low speed I'll worrk about top end later.

    Could either of these things below be my culprit?

    Since I took the CEN and MAG heads off to check piston wash and reused those head gaskets and did not use copper spray, could I just be getting a little water leaking in the center and mag cylinders and when I choke it the extra fuel helps? I'm not seeing any water droplets.

    Could the 80 gram black springs be giving me the issues I'm describing?

    How about the carb to intake gaskets? They are not new but are not torn or anything either, should I try some grease on them and re-install?

    The MAG cylinder is definitely warmer, but I can still touch it without burning myself, so I'm not scared of burning it down at this point.

    The plugs actually look like plenty rich, they are not light tan or white like a lean cylinder would indicate.

    Any thoughts are appreciated. Still have a few warmer days ahead here in MN, so looking to get it "close to tuned in" before it gets too cold.


  2. #2
    Banned User
    Join Date
    Jun 2010
    Milwaukee WI
    Did you check/clean or replace the fuel selector valve?

  3. #3
    Took it apart cleaned, put a little grease on the o rings and put it back together. I have another identical ski that runs perfect, if you think that could be it, I can swap it and try that...

  4. #4
    Click avatar for tech links/info, donation request K447's Avatar
    Join Date
    Jul 2007
    near Toronto, Canada
    Quote Originally Posted by Superbawn View Post
    Took it apart cleaned, put a little grease on the o rings and put it back together. I have another identical ski that runs perfect, if you think that could be it, I can swap it and try that...
    Did you replace the o-ring?

    Typically the original o-ring inside the metal fuel selector valve is dried out from age and if you look carefully it has surface cracks, no good anymore.

  5. #5
    BlueFishCrisis's Avatar
    Join Date
    Aug 2011
    Medina, Ohio, United States
    You can temporarily bypass the valve if you suspect it.

    How are you checking the plugs? They should be checked while running at full throttle. Idling back to the ramp or dock will usually make them look rich. While running WOT, pull the lanyard killing the ski. After you coast to a stop, remove the seat, pull and check the plugs.

    You can also take a look at the piston tops through the spark plug hole. If you have water intrusion, the tops or centers of the pistons will appear cleaner that the outside of the piston. If the piston is all black edge to edge you are lean.

    Did you update your tank vents to make sure they are letting expanding vapors out and fresh air in as fuel is consumed>

    Are you sure you are getting spark to all three plugs all the time?

    How long is the pulse line to the fuel pump? Should be less than 12".

    Did you clean out / check the pulse fitting on the engine block to be certain it is clean and clear?

    What kind of fuel lines are you using? I have had bad luck with the clear lines / tygon lines when utilizing the stock hose clamps. The OD on the line is a good amount smaller than standard 1/4" fuel tubing causing awkward clamping force and allowing air to leak.

    What is the condition of your fuel water separator and O ring. Also need to be in good condition.

  6. #6
    I'm still breaking the new piston in, so I haven't held it wide open yet. I've just been idling out, bliping the throttle for 4-5 minutes, coming back, letting it cool down etc. I'm at the point of my break in process where I can run it for 5 minute or so varying the throttle at midrange, so the plug color really would not be accurate right now for a wide open scenario. It's tough to see the piston wash..I need to find a good little pen lignt or something. Even though I'm not seeing any air bubbles in the gas lines I still think you could be right about the gas lines, I don't like the way these clear blue lines clamp down either. I just found one that would seep a little air bubble in if I moved it, so I moved the clamp over the big part of the fitting and it stopped. I think you're onto something there with those lines and I think I might move the clamps first and if thats the culprit, replace with the black hose. The pulse line is short, maybe 10-12", but I did not ensure the fitting was clear of any obstructions (AARRRG!) but that could be a big possibility since that is the piston that blew the hole and thats where the pulse fitting is. Only thing I can do now is poke something in there I guess to clear the hole if anything is lodged in there and hope it's not a big chunk thats going to rattle around in there when I start it. Expansion vent works when I suck on it, inlet vent also works. Spark on all 3 at the same time. O-Ring on separator in good shape.

  7. #7
    bowsniper's Avatar
    Join Date
    Aug 2008
    Check the fuel lines in the tank if so equipped. Some may have a fuel sender instead. . If you have 2 lines in there.....replace them. Measure each one. The longer is your reserve. There should be weighted screens on them also.

    Clean the tank out. Rebuild the carbs with oem carb kits including the diaphrams. Then change the fuel lines and fuel filters. Then adjust to fac tory specs. Keep the restrictor from the return line and put it back in the new line.

    Replace fuel pump with triple oulet type and install as per beerdarts fuel routing diagram. Should be good to go after that.

    While in the carbs.....find out what jets you have in the mains and pilots. They are under the metering block.

    Just my 2 cents.

  8. #8
    bowsniper, all of that stuff has been done. I am going to pick up some new 1/4" black fuel hose to replace the clear blue stuff I put on because you can't get it clamped down properly.

  9. #9
    Polarisitis loonatik's Avatar
    Join Date
    May 2010
    Tampa, FL
    For trouble shooting purpose, you can temporarily use a 4 or 6 inch zip tie on those blue hoses.

    2.0 needle and 80 gr spring actually put the POP at 21 psi, specs for your ski is 10 - 18 psi.
    To compensate for this, low screw should be turn out a bit, it may even be a whole turn or more. If my rough figure is right, your low screw should be around 1-1/2 to 2 turns out to get this ski running properly.

  10. #10
    bowsniper's Avatar
    Join Date
    Aug 2008
    Ohok. Good deal. Sounds like you have it all updated. Where are you now on it? There's always a possibility that you have some water/ oil/ gas in the diaphrams and they are not pumping gas. They have to have just air on the 1 side to compress and operate the fuel pump. I had it happen before. Once I drained them, ski ran fine after. Just loosen the screws on the diaphram covers and see if you get any fluid leaking out.

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