CV Carb Tuning
Factory Pro Tuning
Follow steps in order....,
1. Top end (full throttle / 7.5k to redline) -
Best Main Jet must be selected before starting step 2!
Best Main Jet
2. Midrange (full throttle
Select best needle clip position
3. Low end (full throttle / 2k-3k)
Float height (AKA fuel level &
- To get best low-end power, set float height (fuel level) so that the
engine will accept full throttle, without missing or stumbling, in 2nd gear
from 2.5k to 3k rpm at minimum.
- Float heights, unless otherwise specified in the installation guide,
are measured from the "gasket surface" of the carb body to the highest part
of the top of the float - with the float tang touching but not
compressing the float valve spring.
- If the engine has a "wet" rhythmic, soggy area at full throttle /
3k-4k rpm, that gets worse as the engine heats up, lower the fuel level by
resetting the float height 1mm greater (if the original was 13mm - go to
14mm). This will lower the fuel level, making full throttle / 2k-3k rpm
- If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel
- Example: change float height from 15mm to 14mm to richen up that
REMEMBER, since the main jet WILL affect low speed operation, the MAIN
JET has to be within 1 or 2 sizes of correct before final float setting.
WARNING: If the engine is left with the fuel level too high,, the engine
may foul plugs on the street and will be "soft" and boggy at part throttle
operation. Adjust Floats to raise/ lower the Fuel Level.
- Base settings are usually given if a particular application has a
history of fuel level criticalness. The Fuel level height in the float bowl
affects full throttle/low rpm and, also, richness or leanness at cruise/low
- Reference: a bike that runs cleanly at small throttle openings when
cold, but starts to show signs of richness as it heats up to full operating
temperature, will usually be leaned out enough to be correct if the fuel level
is LOWERED 1mm. Check out and RESET all: Suzuki (all), Yamaha (all) and
Kawasaki (if low speed problems occur). Needless to say, FUEL LEVEL IS
- If there are low-end richness problems, even after lowering the fuel
level much more than 1.5mm from our initial settings, check for needle wear
and needle jet (part of the emulsion tube). It is VERY common for the
brass needle jets (in the top of the "emulsion tube")
in 36mm, 38mm and 40mm Mikuni CV carbs to wear out in as little as 5,000
miles. Check them for "oblong" wear - the needle jet orifice starts out round!
4. Idle and low rpm cruise
Fuel Screw setting (AKA mixture screws)
- There is usually a machined brass or aluminum plug over
the fuel screws. It's about the diameter of a pencil. The plug must be
removed before proceeding (click
here for instructions).
- Set for smoothest idle and 2nd gear, 4k rpm, steady state cruise
operation. Set mixture screws at recommended settings, as a starting point.
For smoothest idle, 2nd gear 4000 rpm steady state cruise , and 1/8 throttle
high rpm operation.
- Pilot fuel mixture screw settings, float level (but, you've "fixed"
the fuel level in Step 3 above - which you have already done!) AND pilot jet
size are the primary sources of mixture delivery during 4000 rpm steady
state cruise operation.
- If lean surging is encountered, richen mixture screws (turn out) in
1/2 turn increments. Alternative pilot jets are supplied when normally
- Pilot fuel mixture screw settings, float level and pilot jet size
also affect high-rpm, 0 to 1/8 throttle maneuvers. Too lean, will cause
surging problems when the engine is operated at high rpm at small throttle
openings! Opening the mixture screws and/or increasing pilot jet size will
usually cure the problem.
NOTE: A rich problem gets worse as the engine heats up.
- If the throttle is lightly "blipped" at idle, and the rpm drops below
the set idle speed, then rises up to the set idle speed, the low speed
mixture screws are probably set too rich: try 1/2 turn in, to lean the idle
NOTE: A lean problem gets better as the engine heats up.
- If the throttle is lightly "blipped" at idle, and the rpm "hangs up"
before dropping to the set idle speed, and there are no intake leaks and the
idle speed is set at less than 1000 rpm, the mixture screws are probably too
lean: try 1/2 turn out, to richen mixture. Be sure there are no intake leaks
and the idle speed is set at less than 1000 rpm!