Author Topic: Mikuni Flat Side Carb Tuning for Dummies  (Read 36573 times)


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Mikuni Flat Side Carb Tuning for Dummies
« on: January 12, 2012, 05:20:24 PM »
Hi All! After a bazillion hours trying to learn how to tune a carb I came up with this simple outline I could follow. Maybe it could help others.
Believe me it is written by a Dummy, me! LOL
                 TUNING The 2-stroke Slide Valve Mikuni Carb:

      These Carb Setup Procedures ASSUME that the operator
     has performed the following "pre carb setup steps":
     (I know, a big assumption) but anyone who skips these
     "pre carb setup steps" is only making things hard on
     themselves at best and may cost themselves ALOT of wasted
     time and risk major MONEY outlays for damaged equipment later!
     Good/fresh premixed fuel, new/properly gapped plug, clean
     fuel tank/lines/fuel filter, well operating fuel pump if present,
     clean air filter, and most importnt for these procedures a
     carb that has been COMPLETELY taken apart, each part and the
     carb body thoroughly cleaned and all cleaned out with high pressure
      It is strongly recommended that some sort of Carb Tuning Notebook
     be started (if not already done). After each step, write down:
     any weather data present (temp, altitude, humidity, etc) and the
     settings from each SYSTEM you end up with. This will make all future
     carb tuning SO MUCH easier and QUICKER. Also, for those that travel
     with their toys that require different settings for best performance
     it then becomes easy and quick to set up for trips.
      These Carb Setup Procedures contain Six Distinct CARB SYSTEMS
     that control 2-stroke fuel/air mixtures:

   1. Fuel Inlet system (Float/Fuel Level)-
   2. Rich Mixture Start system -                     CHOKE
   3. Pilot/Air System -               IDLE to 1/4 THROTTLE
   4. Needle/Jet System-       1/4 THROTTLE to 3/4 THROTTLE
   5. Throttle Slide Cutaway-  1/4 THROTTLE to 3/4 THROTTLE
   6. Main Jet System-                        FULL THROTTLE

      At a minimum, which is during the initial carb SETUP,
     the STEPS listed below MUST MUST be done in order,
     without skipping any steps.

            (STEP 1)  Float / Fuel Level System:

       The LEVEL of FUEL in the carb float BOWL establishes
      a base of fuel available for the other 6 systems to
      use for the carb to function properly. Because (once the
      fuel level is set the first time) the fuel level remains
      unchanged (unless the float becomes inoperative)
      many backyard mechanics skip setting the fuel level for
      the carb.
       EVERY type carb installation has a fuel level
      specification, which MUST be setup ONE TIME to enable
      all the other carb functions to be correct.
      What basically happens in this first system (assuming
      fuel tank/fuel lines/fuel filter/fuel pump are clean and
      free flowing) is that fuel flows into the carb bowl until
      the fuel level raises the twin floats up to a point where
      the lever closes an inlet valve, shutting off the flow of
      fuel. The exact spec for setting the bowl fuel level varies
      from machine to machine due to differences in motor mounting
      angles, so the exact same carb fuel float level setting
      cannot be the same from machine to machine UNLESS the motor
      angles are EXACTLY the same.
       However, the way to measure the fuel level is the same from
      machine to machine, a millimeter measurement is taken from
      the TOP EDGE of the CARB BODY (with the carb bowl removed)
      to the point on the FLOAT LEVER where the PINS CONTACT.
      This measurement is ALWAYS taken with the carb in an
      **** The fuel level is adjusted by BENDING the SMALL TAB that
      actuates the inlet valve. Once set correctly, this step
      usually remains stable on the carb.
       TROUBLE SHOOTING for this correct setting is easy, if the
      carb seems to "run out of fuel" under FULL thottle (or/and)
      the carb "loads up with fuel" after  the machine has been
      parked for awhile, then the carb float level INLET VAVLE
      has probable failed.
      To preclude this type of trouble, it is a very good idea
      to replace the carb float INLET VALVE at least annually.

           (STEP 2)  Rich Mixture Start System:     

       A spring loaded plunger is used on the carb to open and
      close a straight through fuel passage that provides
      super-rich fuel for 2-cycle COLD STARTING. The plungers
      actuating cable or lever must be adjusted so the plunger
      bottoms out in the OFF position while completely uncovering
      the fuel passage in the ON position.
       The Mikuni VM uses a special enriching circuit that employs
      motor back pressure from the intake tract to literally pump
      fuel into the motor in a separate circuit. While this system
      is not as tunable as the other 6 systems, there are a couple
      performance factors that must be looked at in our normal
      operating environments.
       MOISTURE is one of these factors, especially in cold weather.
      Moisture (water) gets past the rubber cap on the plunger and
      freezes, then when the plunger/choke is used the next time
      one of two bad things may happen. The ice can prevent full
      choke opening, disarming the choke enriching system or/and
      the operator in trying to get the choke to open fully torques
      the plunger too hard and internal part fail. Smaller amounts
      of moisture can cause the plunger to "stick" in the open
      position causing incredible low-speed rich mixture problems.
      To preclude this issue, regular replacement of the plungers
      top seal is recommended.
               (STEP 3)   Pilot Jet / Air Screw System:

       This system has two separate parts. A fixed FUEL JET and
      a spring screw variable AIR JET. This system is most often
      adjusted using the spring AIR JET. A good starting point
      for the AIR JET is 1 1/2 turns out. Most often, final spring
      AIR JET adjustment ends up between 1 turn and 2 1/2 turns
      out. Less than 1 turn and more than 2 1/2 turns normally
      means the fixed FUEL JET needs replaced. 
       Setting the Pilot Jet / Air Screw begins with having the
      motor at operating temperature. After warm up, set the idle
      speed about 500 rpm higher than the normal 2200-2400
      rpm idle speed. Adjust the spring AIR SCREW in 1/4 turn steps
      in/out until the maximun idle speed achievable is found.
       Reset the idle speed back to the normal 2200-2400 rpm.
      Repete the first step again and adjust the spring AIR SCREW in
      1/4 turn steps in/out until the maximun idle speed achievable
      is found. If the idle speed stays within the normal idle 2200-2400
      rpm range, you are done.
       However, if the idle speed climbs past the 2200-2400 rpm range,
      you must again reset the idle speed to normal 2200-2400 rpm
      and repeat the 1/4 turn spring AIR SCREW adjustments to reach the
      2200-2400 range.
       Once the setting is finally reached of 2200-2400 rpm, turn off
      the motor and count the spring AIR SCREW turns IN until it gently
      seats. If the turn in count is between 1 and 2 1/2 turns all is well
      and this adjustment is finished.
       However, if the turn in count of the spring AIR SCREW is less than
      1 turns or more than 2 1/2 turns you now know the FIXED FUEL jet
      needs replaced. You must first replace the FIXED FUEL jet and
      repeat the spring AIR SCREW adjustments.
       If the spring AIR SCREW turns in less than 1 turns a LARGER
      FIXED FUEL jet must be installed, if the spring AIR SCREW turns
      in more than 2 1/2 turns then a SMALLER FIXED FUEL jet must be
       IMPORTANT NOTE. The Pilot Jet / Air Screw System directly affects
      the motors pickup off idle when the throttle is abruptly opened,
      Which in off road riding happens ALOT. So any tuning doubts
      should be towards a RICHER setting. Remember that in adjusting the
      spring AIR SCREW we are adjusting AIR not FUEL. So RICHER means
      turning the spring AIR SCREW "IN" or clockwise, turning the spring
      AIR SCREW out LEANS the mixture.

                (STEP 4)  Needle/Jet System:

       Because this system is directly connected to the Throttle Slide,
      moving as the throttle is opened, the Needle/Jet System is the
      most flexible jetting on the Mikuni VM. It operates over the
      widest range of throttle settings, from 1/4 open to 3/4 open.
      of constantly varying the mixture of incoming air (controlled
      by the throttle NEEDLE SLIDE) and constantly metering the fuel fed through
      the main FUEL NOZZLE.
       The JET NEEDLE is a tapered rod that is pulled out of the
      NEEDLE JET as the throttle is opened and the THROTTLE SLIDE
      is pulled out of the VENTURI. Fitted with grooves along its
      top, the exact position of the NEEDLE JET is varied to control
      the fuel/air mixture. The NEEDLE JET is positioned by a
      CIRCLIP that secures the NEEDLE JET to the NEEDLE SLIDE.
       Exact NEEDLE JET position can be raised (richer) or lowered
      (leaner) to fine-tune MID-THROTTLE response by moving the
      CIRCLIP. 90% of all MID-THROTTLE tuning is done by moving the
       TUNING the motors mid-range performance is done by controling 3
      things for the NEEDLE JET:
       1. position of the CIRCLIP (up the NEEDLE JET for richer and
           down the NEEDLE JET for leaner)
       2. changing the NEEDLE JET length
       3. changing the NEEDLE JET tapers
           (each jet has 2 taper factors; taper at the top of the jet
            and taper at the bottom of the jet)
       Each NEEDLE JET is identified by the letters and numbers stamped
      on them. Example: (6FH7 stamped on the NEEDLE JET)
       6 is the length, which in this case is more than 60mm
         but less than 70mm
       F is the top taper, A-E having less taper
       H is the bottom taper
         (sometimes the NEEDLE JET has only one letter, in that
          case it refers to the bottom taper)
       7 is a mfg code not normally used in tuning.
        Tuning adjustments needed near the 1/4 throttle position
      are done by changing to a NEEDLE JET with a different
      first letter.
       Tuning adjustments needed near the 3/4 throttle position
      are done by changing to a NEEDLE JET with a different
      second letter.

                (STEP 5)  Throttle Slide Cutaway System:

       On the "fuel/air inlet side" of the carb there is a
      cutaway that permits setting the idle and also controls
      the air ratio changes in the lower throttle settings.
      Numbers on the slide normally range from 0.5 to 4.5
      and step up (or down) in .5 increments. The LARGER
      numbers permit MORE AIR FLOW and therefore represent
      therefore represent a RICHER MIXTURE.
       The THROTTLE SLIDE CUTAWAY System also effects 1/4 to 3/4
      throttle operation as does the NEEDLE/JET System. However,
      the effects felt by the operator and the tuning methods are
      very different. The easiest way to test the throttle slide
      is by testing for ACCELERATION RESPONSE.
        After performing ALL the above steps 1 - 4 in order:
      It is time to operate and test the motor by abruptly opening
      the throttle about to 1/4 throttle. You will experience 1 of 3
      conditions (described in detail below): 1. Bogging, 2. Missing
      /rough running/excessive smoke or 3. good running.
       Make sure the motor is warmed up, abruptly open the throttle
      from idle to 1/4. If the motor appears to "BOG" then a lean
      condition is probably present (too HIGH a number on the THROTTLE
      SLIDE), this can be easily tested by temporarily turning "IN"
      the spring AIR JET to see if the bog goes away. If it does correct
      the "BOG", return the spring AIR JET to its proper place and lower
      the number on the THROTTLE SLIDE. Keep this up until acceptable.
       If the motor "MISSES", runs rough or smokes alot, then the
      THROTTLE SLIDE number is too LOW, causing a too RICH mixture.
      This can be easily tested by temporarily turning "OUT" the spring
      AIR JET, if the motor performance improves you know what to do;
      return the spring AIR SCREW to its proper setting and RAISE
      the number of THROTTLE SLIDE.               
       If the motor appears to work well at this 1/4 throttle test
      Before you do any THROTTLE SLIDE adjustments, proper THROTTLE
      SLIDE adjustment that needs no adjustment can be verified
      easily. Slowly move the THROTTLE AIR SCREW in and out, if this
      has Little effect on the test then the THROTTLE SLIDE is set
      correctly and needs no adjustment.
       In Most Cases, unless an new air leak or a component change has
      been made, the THROTTLE SLIDE needs little maintenance. One of
      biggest motor changes that will require THROTTLE SLIDE changes
      is when the exhaust has been modified/become clogged.

                (STEP 6)  Main Jet System:

       The MAIN JET determines the motor being able to produce the
      largest amount of POWER. Main Jet numbers range from 50 to 500,
      the larger the number the greater the flow of fuel. Main Jet
      numbers 50 to 200 normally change in increments/steps of 3 and
      Main Jet numbers above 200 change in increments/steps of 10.
      The Standard Main Jet size, for example, of the fl250 is 120.
       The MAIN JET is screwed into the main FUEL NOZZLE in the carb
      float bowl and is mostly associated with FULL THROTTLE (WOT).
      However, expert tuners know the Main Jet does affect performance
      beginning at 1/4 throttle. This is because the fuel for midrange
      motor performance MUST go through the MAIN JET NOZZLE first.     
       MAIN JET TUNING is normally required due to ALTITUDE changes.
      The lower Altitudes normally require Lower Numbered MAIN JETS and
      higher Altitudes require Higher Numbered MAIN JETS. In most cases,
      when a motor is being retuned because of Altitude changes only the
      MAIN JET needs changed. Atmospheric changes other than altitude,
      such as seasonal cold climate changing may require a larger MAIN JET.
      Main Jet changes may also be required when motor changes are made,
      such as exhaust changes.
       MAIN JET tuning is done by operating the motor at full throttle on
      a flat, level and hardpacked area. Signs that the MAIN JET needs
      changed are: not reaching normal full speed, motor "popping", excess
      smoking, motor missing, excess heat (compared to heat levels that
      are normal for this type installation / this vehicle. Normally you
      should try a richer (higher number) MAIN JET and keep going up in
      size until you reach the highest MAIN JET number that will give you
      satisfactory performance. The richer the mixture normally equates
      to lower motor operating temps and longer life.
       Remember, both the lower rpm NEEDLE JET and the higher rpm MAIN JET
      share the MAIN JET NOZZLE area. So if you make changes to the
      MAIN JET, especially if they are major changes, you may have to go
      back and retune the NEEDLE JET.


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Re: Mikuni Flat Side Carb Tuning for Dummies
« Reply #1 on: January 16, 2012, 06:58:19 PM »
Dang dude, you just gave yourself a sure fire case of carpal tunnel syndrome with that post!!! You better check and see if the moderators have workman's comp insurance!!!   ;D

Thanks for the info!