Hi All, I am no expert, in fact just learning. In trying to understand tuning carbs, the really great articles on this forum, seemed to suppose that the reader had more upfront knowledge than I had, What I did to give a newbie type step by step carb tuning procedures was compile the following from many many sources. Hope this helps other rookies like myself.

TUNING a 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 air.
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 UPSIDE DOWN POSITION.
**** 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 installed.
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.
The JET VALUE, NEEDLE VALUE and NEEDLE POSITION are capable 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 circlip.
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
a LEANER MIXTURE. The LOWER NUMBERS permit LESS AIR FLOW 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.