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  1. #1
    Tech Guru Resident Guru
    Join Date
    Feb 2002
    Fort Mohave, AZ

    Tech Article : 44Mikuni Carbs – Part Technical …Part Tragedy Part 1

    The 1986 Kawasaki 550 was the first PWC to be equipped with the “round Pump” BN 44mm Mikuni carb. Most Kawasaki shop mechanics were familiar and proficient with the “2 adjuster screw” BN38 Mikunis, but many of those same mechanics struggled to master adjustment of the “3 adjuster” 44 carb…. And with good reason. During that time, I was hired to do R&D for Westcoast Performance, and my main job was to simplify complexities for our customers…. 44 Mikuni adjustment moved quickly to the top of that list. Getting the 3-screw 44 to carburate perfectly was not really tough at all (for me anyway). But very soon I learned the big problem everyone else was having with it. If you just hopped on a 44 round-pump equipped 550 ski and rode it continuously with little or no breaks, it would carburete perfectly all day. However any time you stopped to rest on a beach for 10-15 minutes, the engine was badly loaded up when the boat was restarted. Once you “cleaned it out” the boat was back to running perfectly …. Until the next 10-15 minute stop. This was a particularly big problem for guys who just stopped in open water to take a break. Very often, a 550, flooded in this way, could kill a battery before it got started back up. No amount of screw adjustment (or needle and seat changes) solved this problem…. And 550 owners as well as dealer mechanics were getting very tired of living with the annoying and unsolvable problem.

    At the time, Westcoast wanted to market the 44 carbs, but we first needed this phantom problem fixed. Eventually, Mikuni engineers found the problem. The way the low speed circuits had been drilled, when a running 550 engine was shut off, the entire contents of the carb float chamber could drain into the inlet manifold (it took about 10 minutes)…. Creating a flooded condition on restart….. every time. Armed with this knowledge, Mikuni engineers changed the way the low speed circuits were drilled, and this update eliminated the float chamber draining …. And with that, eliminated the random engine flooding.

    At the time, the only way to tell the new 44s from the old ones was the aluminum fuel input banjo (instead of the earlier plastic banjo). Mikuni painted most of the updated aftermarket carbs for the various distributors like Westcoast. The Westcoast carbs were red … others were blue, grey, etc. Westcoast (and other shops) sold thousands of these new circuit 44 “3 screw” carbs, but we were asked to say nothing about the low speed circuit design change (for fear of triggering an expensive recall of carbs on all 86-88 550s).

    The updated “round-pump” 44s appeared on the very first Yamaha 650 runabouts (LX 2-seater). By then, I had started Group K, and Yamaha offered me an LX to prototype our Sleeper Kit. After a few weeks, the kit was done, and the updated round pump 44 tuned in easily, and ran flawlessly…. The Yamaha guys were anxious because the new (at the time) Sea Doo 580 was much faster that a stock LX650. After our testing, they were very happy because the Sleeper LX was considerably faster than the stock SD580. About a year later, Yamaha released the first SuperJet with the same 650 engine as the LX … but equipped with the brand new (and much hyped up) SBN 44 (square-pump) Mikuni carb. Yamaha offered me a SuperJet (for the same Sleeper Kit testing), and I quickly built the SuperJet Sleeper with the same specs I used on the successful LX 2seater. Unfortunately, the new SBN44 of the SuperJet had a low speed bog that I could not eliminate with screw adjustment or jet change (we tried “everything”). I installed a round body 44 from an LX, and the SuperJet Sleeper suddenly carbureted flawlessly and ran great. I personally knew techs at both Yamaha Motor Corp and Mikuni America, so I started a series of phone calls to find out why on earth this “new” 44 was less effective that the old 3-screw 44.

    Don't forget to read Part 2 of this article.
    Last edited by Group K; 12-15-2017 at 08:44 AM.
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.


  2. #2

    Re: Tech Article : 44Mikuni Carbs – Part Technical …Part Tragedy Part 1

    Interesting article would love to continue reading about your work.

    Absolutely devoured everything on your website when building my 650sx. Your experience was invaluable and I am grateful you share the knowledge especially for us overseas

  3. #3
    I dream skis JC-SuperJet's Avatar
    Join Date
    Jul 2013
    Cypress, TX

    Re: Tech Article : 44Mikuni Carbs – Part Technical …Part Tragedy Part 1

    Cool story

  4. #4
    I dream skis Firebird A/C&Heating's Avatar
    Join Date
    Mar 2003
    Phoenix Metro Area

    Re: Tech Article : 44Mikuni Carbs – Part Technical …Part Tragedy Part 1

    X 2
    Last edited by Firebird A/C&Heating; 12-12-2017 at 09:13 PM.
    88 bullet reed 550.. if your 550 can beat it.. you can have mine.
    03 SXR 800

  5. #5
    PWCToday Regular
    Join Date
    Feb 2016
    New Jersey

    Re: Tech Article : 44Mikuni Carbs – Part Technical …Part Tragedy Part 1

    Please keep these articles coming. Thanks for sharing

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