» Log in

User Name:


Not a member yet?
Register Now!

» Find OEM Parts

» Jobs

» wallpapers

Mikuni SBN Carb Tuning

Printable View

  • 07-15-2010, 07:18 PM
    Mikuni SBN Carb Tuning
    This is for tuning a SBN carb for a modified motor. If you motor is stock, you should be using the stock settings on your HS and LS screws. You can use this to make minor adjustments due to elevation or compression. If you have a stock motor and it is not running properly, this procedure will not help you.

    Disclaimer from Bill O about these instructions:
    “Generally speaking, most Mikuni's share very similar tuning instructions, with only minor differences.
    I will tell you this, you will not be an expert carb tuner from reading someone's instructions. Only experience will give you real knowledge of tuning.”

    This is a combination of multiple sources from this site for older models of two strokes. This is not a definitive all in one guide. Big thank to Bill O’Neal and all of the contributors on this site. If you really want to dig in, there are links at the bottom to the Mikuni manual and other good procedures.


    Rich: Too much gas
    rich hesitation is when the ski seems to go lower in RPM's and slowly comes back to life. BBrrrrrrrrrrraaaaaaaapppp!

    Lean: Not enough gas
    Lean hesitation is sounds like the ski dies for a split second and comes back to life. BB__rraapppp!

    Bogequals a rich condition; symptoms of a bog are that the motor will not rev up right away because it needs to burn off excess fuel that is being put into the motor because the low speed adjusters are too far opened. If you were to let it idle for a minute or two, the sparkplugs would look a bit wet with fuel if you were to remove them without revving up the motor first. If the engine takes a second or two to clear out and then accelerate, the carb is too rich.

    Hesitation. A lean hesitation happens when you stab the throttle from idle, and it acts like it is taking a huge deep breath before revving up, or it just completely dies for lack of fuel. The latter would be a very lean condition probably caused by the accelerator pump not working correctly, or the low speed adjusters are turned way too far in.
    If the engine hesitates, the carb is lean.


    Before you start.
    Start rich!! start 2 turns out on both and see how the boat reacts. You are going to have to play with it because regardless what the book says you have a lot of different factors. Elevation, compression, etc. Every boat will tune different and a similar boat will have different ideal settings.

    Before attempting to tune, always clip the sparkplug wires to make sure you have a good connection at the sparkplug boots on the wires. You also should know that the motor has good compression because you cannot tune a junk motor.

    You cannot properly tune an engine that has low compression in one or both cylinders.

    Carburetors are a compromise at best, and tuning them for anything EXCEPT idling and full throttle use is always a compromise.

    Can I ruin my motor doing this? YES!

    A rich motor will run terrible, but will not kill your motor.
    A lean motor will kill your motor.

    How can you tell if it’s too lean?
    Most novice tuners cannot properly read sparkplugs.

    A good indication of a lean motor is if the sparkplugs get so hot you cannot lay them on your opened hand without burning yourself. So when you pull your sparkplugs, do they feel real hot, or do they feel normally hot? Really hot is an indication of a slightly too lean motor and it takes no plug reading skills or special tools or instruments.

    More plug reading tips below.
  • 07-15-2010, 07:18 PM
    1 Attachment(s)
    Re: Mikuni SBN Carb Tuning
    [ ] = directly from SBN manual

    This is written for your boat being in the water. You need the load of the water for accurate tuning. Stay close to shore and don’t get too far out.

    Mikuni says adjust your carb in this order: Do not go to the next step if the current step fails.

    1. LOW SPEED ADJUSTER -To adjust a smooth idle ONLY
    2. POP-OFF PRESSURE (should be set from carb build already. adjust if you can’t get low speed jet to work)
    3. LOW SPEED JET – idle to midrange
    4. HIGH SPEED JET – midrange to 3/4
    5. HIGH SPEED ADJUSTER – 34/ to WOT (wide open throttle)
    Note: Should never jump to step 4 before 3 is set.

    [The reason for adjusting the circuits in this order is because several circuits contribute to the total fuel delivery of the carb. Changing the low speed jet for example, affects wide open throttle fuel delivery to some degree. The exceptions to the rule are the low speed adjuster and the regulator portion: the low speed
    adjuster has no effect past 1/3 throttle. The regulator portion has no tuning effect past 1/4 throttle, although it continues to control the fuel supply]


    To properly tune carbs you must first get the idle speed corrected so that the motor idles at the right rpm, or very close to it. [Set idle speed 1100]-1500

    [The low speed adjuster is used in conjunction with the idle stop screw to adjust and maintain idle speed and smoothness. Experiment turning the low speed adjuster in and out in small increments until a smooth idle is obtained. As the idle stop screw is turned in our out to raise or lower idle speed the low speed mixture is also affected. For clarification, if the idle stop screw is turned out to lower idle speed, this action increases manifold pressure slightly and richens the low speed mixture so that a mixture adjustment may be required. The low speed adjuster is very sensitive and adjustments should be made in small increments only.

    Note: Remember, the low speed adjuster is only for adjusting the idle mixture. If you use the adjuster to help get rid of a low speed hesitation, you will probably find that your engine will load up in no wake zones, or after extended idling]

    [For clarification, if the idle stop screw is turned out to lower idle speed, this action increases manifold pressure slightly and richens the low speed mixture so that a mixture adjustment may be required.]

    You want the thing to idle right without loading up and fouling out your sparkplugs at idle, they will never look as good as they will after a wide open throttle run if you let it idle for a minute or more, but they should not be wet and dark black sooty looking or anything like that. Dark brown at idle is OK. Wet and sloppy is not.

    You must get it to accelerate from idle speed to the midrange using the leanest possible adjustments for the low speed adjuster. If you cannot achieve that, your low speed pilot jet is either too small or too big, or your pop off pressure is too low or too high, or a combination of both, not jetted correctly, and too high or too low pop pressure

    2. [Pop off pressure checked in rebuild]

    [Ride the boat at a constant 1/4 throttle opening for about 1 minute and then quickly open the throttle fully, there should be no hesitation and the engine should not show signs of being loaded up. If it hesitates, it's lean; if it's loaded up, it's rich. The first test is to check pop-off pressure, the second test is checking the correctness of the low speed jet size. Take the time to ride the boat slowly and thoroughly test your jetting changes. After a jet change, it takes the engine a few minutes of use to completely respond to the change.]

    Do not go to step 4 until you have 1-3 working perfect

    [The procedure for testing for the correct high speed jet size is the same as for the low speed, except that you should now hold the throttle at a constant ˝ open for one minute, then quickly open the throttle fully to check engine response (and release throttle. DO NOT RUN AT WOT yet. These are just short bursts.) If the engine hesitates, the carb is lean. If the engine takes a second or two to clear out and then accelerate, the carb is too rich. In either case, make the appropriate jet change and do the complete test again.

    The high speed jet begins contributing fuel at about 3/8 throttle, overlapping the low speed jet. The high speed jet is the primary tuning component from ˝ to 3/4 throttle. As you have probably noticed, tuning circuit operations are denoted in fractions of throttle openings.. the reason for this is simple: Carb jetting does not relate to engine rpm or the boat's speed, it only recognizes how far the throttle has been opened; each circuit of the carb responds in turn. This is why it's very
    important, when trying to diagnose a carb problem, that you identify at which throttle opening the problem occurs, in order to adjust the appropriate circuit.]

    [The high speed adjuster is the last circuit to adjust. It primarily controls fuel delivery from 3/4 throttle to wide open throttle. Turning the screw clockwise reduces fuel flow, counter clockwise increases fuel flow. The maximum fuel flow is achieved at three turns out from closed. To test the high speed adjuster it is recommended that you start with a fresh set of spark plugs to get quicker plug readings. Unless you have an exhaust gas temperature gauge, you will have to rely on plug readings.]

    [Start rich on the HS to be safe]

    Now you are ready to do your first very short [about 10 seconds] high speed wide open run where you will pull the lanyard off while still holding wide open throttle. Raise the hood or seat and remove the sparkplugs and look at them. If you see any shades of color on anything on the sparkplug that resembles any shade of tan or brown, your jetting is very close to being very good if not perfect. After riding for an hour or two, the rest of the sparkplug will gain some color.

    If they are very dry, you are not getting enough fuel. Something is assembled wrong or something is clogged up, like the brass jets, internal fuel filters or the fuel selector valve.

    No color is too lean, very dark is too rich. Adjust HS and repeat step 5

    Nice brown

    The ultimate color will usually be a light tan to brown color, but not completely white (which can indicate either a rich or a lean condition, depending upon whether the sparkplug is wet with fuel or dry from not enough fuel).
    If you see anything that looks like pure aluminum color, immediately go back to the trailer and find your air leak or fuel delivery problem and fix it. It could be anything from a carb base gasket, case seal, rave valve pulse line split to off the fitting on the cases, a clogged up jet, air sucking into the fuel lines or whatever. It will not be because your jetting is too lean.

    [Another indicator of proper adjustment is a maximum rpm reading on a tachometer. If the carb is lean or rich, it won't pull as high an rpm reading as when it's right on.]
  • 07-15-2010, 07:24 PM
    Re: Mikuni SBN Carb Tuning
  • 07-17-2010, 11:21 PM
    Re: Mikuni SBN Carb Tuning
    Thats great help thanks for the great info.
Powered by vBadvanced CMPS v4.2.0