pxctoday

» Log in

User Name:

Password:

Not a member yet?
Register Now!

» Find OEM Parts

» Jobs

» wallpapers

Results 1 to 5 of 5
  1. #1
    Tech Guru Resident Guru
    Join Date
    Feb 2002
    Location
    Fort Mohave, AZ
    Age
    63
    Posts
    1,152

    Tech Article Mikuni 44 Carburetors Part Technical .. Part Tragedy Part 2

    Signal and the G-Test machine

    In the early 1990s, there was growing conversation among R&D engine guys about inlet signal, and how it affected jetting and carburetion…. But what is signal?

    For many years it was believed (by many engine builders) that air passing through the carb mouth pulled up fuel from the carb circuits to bring fuel/air mixture to the lower end …. Wrong. What actually happens is that the crankcase of a 2 stroke sends a “negative pressure wave” (engineers hate it when you call it a vacuum … but it’s a vacuum) up the inlet port … and that negative pressure wave pulls air through the carb mouth, as well as drawing fuel through the carburetor jetting circuits. The important part of this is that some carbs “see” this negative pressure wave with much better sensitivity than others. This sensitivity is a function of atomizer design, and the relative distances between the atomizer, butterfly, and low speed transition circuits (in the floor of the carb throat). The reason the round pump 44 worked so much better on our Yamaha 650 Sleeper was because the signal sensitivity (aka “signal) of the round pump 44 was markedly better than the newer SBN44. Sadly, knowing this is of no value unless you had a tool that helps work around it …. And Mikuni America had such a tool…. The G Test Machine.

    In the early 90’s, Mikuni America’s head tech was Herb Kane. Herb was aware of the signal “issues” of the new SBN44, but he was confident their new G Test Machine could help find the solutions. I often spoke to Herb during that era, and he spoke with excitement during the construction of the G Test machine. The truth is, it wasn’t a machine at all…it was a 20x20 temperature controlled room built in a corner of the expansive Mikuni warehouse in Northridge California. The room was filled with computer interfaced gauges and meters. It was like a monstrous Flow Bench on steroids. The thing that set the G Test apart from flow benches was that (during tests) it also passed an inert fluid (with the same specific gravity as fuel) through all of the carburetor jets. The machine mimicked the crankcase signal of a two-stroke, and it measured air/fuel mixture flow at every throttle opening in 5% increments. It bears noting that the G Test machine required that the carb be mounted on a complete inlet manifold with reed cages attached (to properly mimic reaction to crankcase signal). A special adaptor plate was made to accept every intake assembly. Next to the G Test room was a bank of huge fans and pumps (the size of a small house) that provided all the air and fluid movement needed to make the machine work. In short, you could install a carb that you knew to work perfectly (like my round pump 44), and the G Test machine would measure and plot the fuel/air flow data at every throttle opening. With that data, you could install another carburetor (that had different signal characteristics)… and tune it (with jetting & adjustments) until the flow data matched. To say the least, I was floored by what it could do … and the precision with which it worked. With this machine, you could do, in hours, the same work that would take weeks of on-water carb tuning. Herb was quick to mention that there were only two G Test machines on earth … one at Mikuni headquarters in Japan … and this one. He also mentioned that the corporate brass was not happy about the cost (which went very much over budget). They were anxious to put it to work to recover costs.

    With respect to my SuperJet Sleeper, I just wanted to replace the troublesome SBN44. I wanted to buy round pump 44s to put on my Yamaha kits, but Herb said that corporate was phasing out all round pump carbs, and the SBNs would just have to be made to work. With that I built a Yamaha 650 inlet system mock up to test the “happy” 44 round pump carb vs. the troublesome SBN 44, and I delivered the whole mess to Herb at Mikuni. A few days later, Herb called to tell me the SBN was ready….I was very apprehensive. As I picked up the carbs, Herb said that I would not need to touch the adjusters … just bolt on and go …. I was even more apprehensive. The next day I went to the water to find that the SBN44 I struggled with so much …carbureted flawlessly …I was impressed.

    In the meantime, I had been testing with the new SBN44 on piston port and reed 550s, as well as Kaw 650s. While they worked well at 60-100% throttle, there were numerous hesitations and "lazy spots" in the 0-50% throttle range that I could not resolve with jetting, adjustments, etc. In earlier conversations, Herb had told me that this was the whole problem with carbs that were less sensitive to "seeing signal".... their biggest problem is delivering "consistant" low speed metering. In addition to this, these same "poor signal" carbs are very much affected by changes in temperature and altitude.
    With that, I called Herb to say that I wanted to build test-bed inlet systems for piston-port & reed 550s as well as Kaw 650s & 750s. I explained that getting all the SBN44 data for these models would result in huge SBN44 sales…. But Herb had a complication. News of new R&D devices, like the G Test Machine, moves quickly among the Southern California motorcycle race teams & R&D departments, and the Mikuni upstairs executives quickly promised all kinds of G Test time to those guys. I told Herb it would take time to build all the PWC test inlet systems anyway… so I could wait. In the coming months my testing got bumped back time and again. Finally Herb committed to giving me a week in late January, which worked great for me (even in Southern California, on-water carb tuning in January is a “chilly” experience) … and then it happened.

    At 4:30pm on 1/17/94, Southern California experienced the 6.7 magnitude “Northridge” earthquake…..freeway interchanges fell to the ground. Mikuni HQ was virtually sitting on the earthquake epi-center. The entire Mikuni headquarters building and warehouse was destroyed … and the G Test Machine was reduced to a pile of rubble. Mikuni America already owned another vacant commercial property nearby, so they immediately began the move to the new site. Herb tried to salvage the G Test Machine, but the destruction was absolute. The upstairs executives made it clear that they would not pay for another G Test machine …. And so that was that.

    With the G Test Machine gone, me (and Herb at Mikuni) were back to solving these problems the "old fashioned" way .... on-water testing. By then, Mikuni Japan was producing SBN44s with different transition circuit layouts ... and this helped a little .... but new transition circuits were not going to fix the basic signal sensitivity issues.
    During this same time, there were a number of aftermarket shops modifying the carb throats and atomizers of the SBN44 in an effort to solve the signal problem. But sadly, in most cases, these modifications resulted in less signal sensitivity ... not more. In time, some of the carb guys "got it", and started to develop throat modifications that actually increased the sensitivity ... and with that, these modified SBN44s were much easier to tune, and they offered excellent 0-60% throttle opening performance. Of all the carb modification shops, the two that succeeded the best at increasing "signal" were Novi, and Full Spectrum. In fact, the signal increase of these carbs was so good that they could increase throat diameters to 46-48mm, and still have enough signal to offer excellent 0-60% throttle response.

    By the time all this carb development was done, everyone had long forgotten about the old round pump BN44. However the fact remains that, despite its lack of changeable jets, it still has very good signal and a broad range of adjustment with it's 3 mixture adjusters. The BN44 still wears a bit of a black eye from the early "draining float bowl" versions ... but at the end of the day, the updated version is not a bad carburetor for piston port 550s, and Kaw single carb 650s.
    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.

    www.groupk.com

  2. #2
    PWCToday.com Is My Home Away From Home beerdart's Avatar
    Join Date
    May 2005
    Location
    CT
    Posts
    2,283

    Re: Tech Article Mikuni 44 Carburetors Part Technical .. Part Tragedy Part 2

    Great story thanks for sharing. My GF now wife was on hwy 14 just north of the overpass collapse that was a scary day ill never forget.

  3. #3
    PWCToday.com Is My Home Away From Home
    Join Date
    Aug 2011
    Location
    Austin, Texas, USA
    Age
    52
    Posts
    6,691

    Re: Tech Article Mikuni 44 Carburetors Part Technical .. Part Tragedy Part 2

    What about the other G Test machine in Japan? Couldn't that have been used for your testing -- although that would have required a trip across the pond...
    Riding mostly Lake Austin

    1984 JS440 -- 1989 650sx -- 1991 X2 -- 1992 750sx -- 1995 900zxi (qty 2) -- 1995 X2

    Quote Originally Posted by cujo View Post
    God intended PWC to be two strokes, as well as dirt bikes, snowmobiles, and chain saws....

  4. #4
    PWCToday Guru
    Join Date
    Mar 2003
    Location
    Shiocton, Wisconsin
    Age
    40
    Posts
    494

    Re: Tech Article Mikuni 44 Carburetors Part Technical .. Part Tragedy Part 2

    "By the time all this carb development was done, everyone had long forgotten about the old round pump BN44. However the fact remains that, despite its lack of changeable jets, it still has very good signal and a broad range of adjustment with it's 3 mixture adjusters. The BN44 still wears a bit of a black eye from the early "draining float bowl" versions ... but at the end of the day, the updated version is not a bad carburetor for piston port 550s, and Kaw single carb 650s."

    Good info Harry....it's funny, every time someone mentions a BN44 around here
    it always ends up with the "throw it in the trash and get a SBN" comment.
    I've had several 650's in the past with WC ready-made BN 44's on them
    and they tuned and ran very well. But some how it appears the PWC world
    lost the ability to tune a BN even though they worked in the past, kind of like
    when the Romans lost the formula for concrete,... it's just gone man. Well, it
    just goes to show you can't believe the hype, a BN can still
    work well for some applications. Just ask Harry, he has the formula!
    Last edited by ACP; 01-14-2018 at 06:21 PM.
    2001 Superjet; R&D, Blowsion, UMI, Riva, Wetwolf, Ocean Pro.

    1987 650sx: Mariner, UMI, Neo Designes, R&D, Jetsports, Renthal, Hydro-Turf, Skat-Trak, Reworked stock pipe, 40mm carb, Wetwolf F/S cone.

    1986 JS300; lots of mods, fast for a 300...I think

    1987 X2: Rips pretty good and has some 80's flare.

  5. #5
    Resident Guru bored&stroked's Avatar
    Join Date
    Dec 2006
    Location
    Mesa AZ
    Posts
    937

    Re: Tech Article Mikuni 44 Carburetors Part Technical .. Part Tragedy Part 2

    Love me some groupK tech articles
    '90 Superjet: ported 760, dual 44's, B pipe, ADA head 190psi, TDR waterbox, hooker 10-16, sponsons. Wife's ride
    '94 Suuuperjet: 84mm 61x, ported, protec pipe, H2o designs RN waterbox, Newmiller head 195psi w/girdle, protec ride plate, Protec modified cdi, UMI grate, Roundnose pole -4", rev footholds, solas prop, sweet green turf+wrap.
    '95 Raider 701: Fiberglass not SMC, Hydrotrailer hitch, ugly purple and yellow everywhere.

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBadvanced CMPS v4.2.0