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Thread: Call out to turbo guys: HSR42 jetting help

  1. #1
    Kaptain Ketchup Guest

    Question Call out to turbo guys: HSR42 jetting help

    Ok. Had the bike out for the first time today on the street. GS1000 with 1100 kit and b25 rajay turbo running a hsr42 drawthrough. Dyna 2000 set at 28 degrees. I was advised to set the carb up with 32.5 pilot jet, Y95 Needle on the richest (lowest clip), and 220ish main jet. The bike ran so rich that it would bog if the throttle was on then let go of too fast, and I didnt need any choke to keep it running from a cold start. Plus my house smelled like gas for a day.. I went south on the pilot jets all the way to a 20 and could still not get the motor idle to change tuning the air screw out. The book says:

    1. If the best idle is achieved with the air
    screw less than one turn out, the pilot jet
    is too small and should be exchanged for
    a larger one.
    2. If the engine speed does not slow after
    two-and-a-half turns out, the pilot jet is
    too large and should be exchanged for a
    smaller one.

    I could not get the idle to slow and thus kept going south on my pilot jets.

    I decided 20 was a bit too small and changed the needle back to the stock Y95 on the middle clip.

    NOW I get the pilot circuit to change the idle with less than 2 1/2 turns out of the air screw.... So I worked my way back to a 25 pilot jet. The air screw is set between the two rough idle points. So I think I am in the ballpark on the pilot jet.

    Now today I went for a ride around the block and the bike backfires through the carb if I roll on a bit quick in second or first (small town didn't get to roll on in third). Yet, I can keep it running smooth if I roll on really really slow...

    I just need some nice staring point jetting numbers for a street turbo. I am thinking the first set of rich numbers I got was for a drag race only bike. I hear these carbs are next to fuel injection in smoothness when jetted properly... I want to get to that point some day.

    A cool harley guy in town here has the mikuni box o' jets that I can trade him for so getting jets would not be a problem. I have read about an fj turbo that had a 17.5 pilot, and a large main (no word on the needle).

    Point me in the right direction... on the up side I can change the jets on a hsr mikuni in record time now!

    Any help is appreciated,

  2. #2
    lecroy Guest


    Your engine may stuble because of the pump. If you disable it or shorten it does it help?

  3. #3
    Join Date
    Jun 2004


    It sounds like you're going about tuning in the "random guess and check" method. Not a sound system. :-) Ignore idle and midrange for now. What you need to do first is to get the main jet sized right. As the main jet affects EVERYTHING else.

    Do wide open runs for 1/2 mile - 1 mile and do and engine cut and plug check. Get those right. Then you can fiddle with the needle to get the midrange right. After all that, then you can address the pilot jet. :-)
    You'd have to be crazy to be sane in this world -Nero
    If you love it, let it go. If it comes back....... You probably highsided.
    1980 GS550E (I swear it's a 550...)
    1982 GS650E (really, it's a 650)
    1983 GS550ES (42mpg again)
    1996 Yamaha WR250 (No, it's not a 4 stroke.)
    1971 Yamaha LT2 (9 horsepower of FURY.)

  4. #4
    Kaptain Ketchup Guest


    Home for the weekend to work on the bike. If i get it running smoothly I can take it with me back to Ottawa on Tuesday.

    Nerobro, can you explain the cut and check method?

  5. #5
    Kaptain Ketchup Guest

    Default I answered my own question.. I think

    The easiest way to determine if you are jetted properly is to do a plug check for each "range" of the carb.
    There are three ranges to adjust in the carb.
    Low speed: Is off idle to 1/4 throttle position. The pilot jet (is inside the carb float bowl and is removable to adjust the mixture by changing the jet to a smaller or larger number size) or fuel screw found under the carb outside between the float bowl and the intake manifold.
    The smaller number on the jet = a leaner mixture. The bigger number on the jet = a richer mixture. The fuel screw turned OUT is richer turned IN is leaner.
    Old time round slide carbs use an air screw (on the outside side of the carb) turning the screw IN richens the mixture and OUT leans the mixture. The air screw is NOT common anymore these days.

    Mid range: Is adjusted by the needle clip and is 1/4 to 3/4 throttle position.
    There are slots on the needle where a clip can be moved up or down to change the mixture. The top position on the needle is leanest (letting the needle stay lower in the bore of the needle jet that it fits into. The lowest position on the needle is the richest letting the needle stay higher in the needle jet bore.

    Top end: 3/4 to wide open throttle is the main jet. It is a removable jet in the float bowl. Just like the pilot jet, it is numbered and the bigger number is richer the smaller is leaner.

    Checking jetting doing a base ring plug check:
    You will need to mark the throttle housing (twist throttle) or remove the top of the thumb throttle cover so that you can watch which part of the throttle opening you are in.
    Using some gray tape place it on the throttle housing so that you can mark the position of the throttle. Mark the sweep of the throttle from closed to wide open. Then divide the sweep into 1/3's. Zero to 1/4, 1/4 to 3/4 and 3/4 to wide open. Then make a mark on the grip or throttle lever inside the housing (thumb throttle) so that you can make note of the position when riding.

    Install a fresh plug and find an area with a slight incline about a football field long.
    You will want to get into 3rd gear and hold the throttle in which ever area of the carb you want to check for the length of the field. Do NOT vary the throttle, you want to hit the kill switch and pull in the clutch, coast to a stop then pull the plug.
    Make note of the plugs base ring color (the area just above the threads, see the link for better pictures)
    A light tan or charcoal color is a safe setting, a cleaner "light tan" is optimum but only for the experienced tuner because you are on the edge of leanness, a dark ring is too rich and the air fuel ratio needs to be changed.
    You can do this check for al three areas of the carb.

    This test will need to idealy be done in 55 to 65 degrees on a not so humid day. Hotter and more humid or colder days will be totally different as far how the jetting will be.
    The hotter it gets outside the richer it will run. The cooler it gets the leaner it will run.
    This should give you the basic understanding of how things work in the carb and how to check the ranges of the carb to be able to adjust the jets or screws.

  6. #6
    Kaptain Ketchup Guest


    Do these sound like good instructions?

  7. #7
    jgordon Guest


    Quote Originally Posted by Kaptain Ketchup
    Do these sound like good instructions?
    My turbo runs great with the jetting about where yours is. Larger jet for more boost. The 210 wold be about right for around 8 lbs so it really depends what kind of boost your going to go. As everyone says you gotta get the main dialed in first and then work you way down.....

  8. #8
    Join Date
    Sep 2004
    Upstate NY


    I'm running a similar setup on an old turbo'd Kawasaki Z1. 1015cc old atp kit with a 42mm flatslide on it. Turbo pistons around 8:1

    needle valve assembly - 2.3 - the 2.3 is designed for use with a fuel pump - the 4.2 is stock...

    the pilot jet - 35 or 37.5 - I've been playing with both - 35 is too lean, 37.5 is too rich

    main jet - 220 - 230 was too rich, although I think 220 is a little too lean

    needle - 8CFY2-95 - clip on 4th slot down

    I run about 7lbs of boost

    I was running 35 pilot with 220 main and an old Gerex ignition - but I switched to a Dyna 2000 running 8 degrees of retard - after I did this, my carburation seemed to go a little lean (not what I expected) - so I've been playing ever since.

  9. #9
    Join Date
    Sep 2004
    Upstate NY


    oh - one change - I switched from B8ES plugs to B9ES plugs.

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