Announcement

Collapse
No announcement yet.

Mikuni 44mm "Round Body" Tuning - HELP

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Mikuni 44mm "Round Body" Tuning - HELP

    Does anyone have any tips to tuning these old-style "Round Body" carbs with the LOW, MID and HIGH Adjustments. I have never felt comfortable with my methods of trying to dial in these carbs. I have done the tie it to the trailer thing and messed with the settings endlessly, however, I never feel like I have it dialed in. The motor and carb are completely rebuilt and fresh, no leaks, correct Pop-off, etc.

    Is their a systematic way of doing this?

    Group K provides some good information, however, they do not get into the process for dialing in the MID adjustment.

    ANYONE HAVE ANY ADVICE, TIPS, SUGGESTIONS, REFERENCE WEBSITES, ETC?
    THANKS

  • #2
    Re: Mikuni 44mm "Round Body" Tuning - HELP

    Hey fleabags - all is not lost - I've got all the tuning info for these things, but due to my schedule, it may take me a couple of days to get it together and posted, so if nobody else steps in , just hang tight, and I'll back with ya -

    Tony
    '88 JS550 with all kinds of "bits and pieces"
    Tony

    1988 JS550
    milled head, bored nozzle, K&N arrestor, rev limiter eliminator kit, PJS Type II Works rear exhaust system, 1/2" cooling system, primer kit, PJS reed pipe and manifold, 15 degree prop, Jetinetics lightweight charging flywheel, trued & welded crankshaft, PJS bulkhead support, PJS speed ride plate

    Comment


    • #3
      Re: Mikuni 44mm "Round Body" Tuning - HELP

      okay, I finally was able to round up the round body Mik tech articles from my collection of vintage mags. It seems even the pros did not agree on the sequence of adjustments - whether to start on the low-speed or the high-speed settings first. One thing that is a fact is that these carbs function differently from "true" carbs - the high-speed fuction is affected by the settings of the pilot, low-speed, pop-off pressure, and needle and seat - not just the high-speed setting.... so if you set your high-speed screw first, you should at least go back and check it again after you've set everything else.

      Starting from scratch on my 550, I set all three screws to the factory recommended settings. For '86 -'88 550's, pilot 1 5/16 turns out, low 1 1/16 turns out, high 15/16 turn out. Take her out, warm her up, and check wide-open-throttle/WOT performance. Come back in, turn the high-speed out a quarter turn. Go back out, see how it feels. Come back in, another quarter-turn out, try her again. Keep doing this until it's clearly too rich (If you can't get it too rich without fear of the high-speed needle falling out, you need a larger needle and seat combo). Now start working your way back leaner, 'til you get it running clean and strong - set it just a touch on the rich side of optimal. If the engine stumbles and feels like it is being "held back" at full throttle, it is too lean.

      Now we'll work on low-speed and pilot screws. For this step, you're going to be continuously working the idle speed screw to keep a decent idle speed. Adjust the low-speed and pilot screws for highest idle speed. Now, take the boat out and check out the throttle response. Accelerate from idle to half-throttle. If you feel a flat spot, like the engine is dying, it's too lean - play around with the screw, a quarter-turn at a time, til it feels good and strong. This is the most time-consuming part for me - it takes me many trips out and back, pulling the hood , tweaking, putting it back on, ... but patience in tweaking will result in a strong, clean-running engine.

      When you're satisfied you've optimized your low-speed settings, go back and check your WOT performance. Ideally, you'll have access to a stretch of calm water, can do a WOT run, kill the motor at the end from WOT - you don't want the motor to run for any period of time after the full-throttle setting - then pull the spark plugs - hopefully, they'll be a beautiful chocolate-brown color.

      Pop-off pressure it is a big factor, but see what kind of results you get from the screws first. To check or tune pop-off pressure, you'll need a specialty tester - a miniature pump/gauge combo. I've had good luck measuring a freshly rebuilt carb on the boat, but for real accuracy or to tune the pressure, you have to disassemble the fuel pump.

      Hope this is of some help, good luck, and let us know how it goes!

      Tony
      Tony

      1988 JS550
      milled head, bored nozzle, K&N arrestor, rev limiter eliminator kit, PJS Type II Works rear exhaust system, 1/2" cooling system, primer kit, PJS reed pipe and manifold, 15 degree prop, Jetinetics lightweight charging flywheel, trued & welded crankshaft, PJS bulkhead support, PJS speed ride plate

      Comment


      • #4
        Re: Mikuni 44mm "Round Body" Tuning - HELP

        prillernut
        Thanks for the info, I am rebuilding the same carb on a friend's 90 550 this weekend and this will come in very handy.

        One thing that is confusing though is you refer to high, low, and pilot. Pilot in your discription is the mid screw?

        Used to working on 4-stroke bikes where the pilot jet and its mixture screw control the idle and progression ports mixture.

        Comment


        • #5
          Re: Mikuni 44mm "Round Body" Tuning - HELP

          Prillernut,

          Thanks a lot for all the time and effort that you put into this. I am going to give it one more try at the lake. If my results are less than satisfactory, then I am going to scrap the BN for an SBN and then have a go out the replaceable jets.

          Thanks again,

          Comment


          • #6
            Re: Mikuni 44mm "Round Body" Tuning - HELP

            hey guys, you're more than welcome! I know it can be very frustrating trying to get good info for a vintage machine - I'm also into vintage motocross bikes...

            The owners and factory service manuals for the JS provide no actual performance tuning procedures, only the prescribed standard settings - they're really only good as a starting point in your tuning. Back in the late '80s - early '90s, we had to rely on the info provided from the magazines - PWI in particular was very good about providing solid, useful tech information.

            hey nvseadoo - (the high-speed and low-speed screws should be marked with single letters, "H" and "L" respectively, on the body of the carb adjacent to the screws). The screw that is referred to as the "pilot screw" is (as you're standing on the starboard side of the boat looking at the carb) on the left/rear side, below the high-speed screw. I don't believe it is marked.

            Hope all goes well, happy skiing!

            Tony

            p.s. - specifically regarding the setting of the pilot screw - the only reference I can recall ever seeing specifically addressing this is in a PWI June 1992 article by George Grabowski, where he says "For those with 44mm carburetors, the adjustments become more difficult because the pilot jet screw must be adjusted along with the high-speed screw."
            Last edited by prillernut; 08-06-2005, 11:11 AM.
            Tony

            1988 JS550
            milled head, bored nozzle, K&N arrestor, rev limiter eliminator kit, PJS Type II Works rear exhaust system, 1/2" cooling system, primer kit, PJS reed pipe and manifold, 15 degree prop, Jetinetics lightweight charging flywheel, trued & welded crankshaft, PJS bulkhead support, PJS speed ride plate

            Comment

            Working...
            X