CV Carb Tuning
for Lower rpm engines
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Follow steps in order...., dial in
the Main Jet.
1. Top end (full
throttle / 5000 rpm to redline) -
Best Main Jet must be
selected before starting step 2 (needle height).
- To get the best, most even top end power (full throttle/after 5000 rpm), select the main
jet that produces the highest top speed / pulls hardest at high rpm.
- If the bike pulls harder at high rpm when cold and less hard when fully warmed up, the
main jet is too large. Install a smaller main jet and retest until you find the main jet
that pulls the hardest at high rpm when fully warmed up. This must be done first - before
moving on to the other tuning ranges.
- If the bike doesn't pull well at high rpm when cold and gets only slightly better when
fully warmed up, the main jet is too small.
- In order to properly tune the midrange and low rpm carburetion, THE MAIN JET MUST FIRST
BE PROPERLY SELECTED after 10 to 15 minutes of hard use!
- Do not pay too much attention to the low-end richness when you are changing main jets -
you still need to be using the main jets that produce the best power at high rpm. You will
deal with the low-end / cruise later - after step 2.
- 2. Midrange (full throttle /2500
to 3500 rpm)
Step 1 (Best
Main Jet) must be selected before starting step 2!
- Select best needle clip position
- To get the best power at full throttle / 2500-3500 rpm, adjust the
needle height, after you have already selected the
best main jet.
- If the engine pulls better or is smoother at full throttle/2500-3500
in a full throttle roll-on starting at <1500 rpm when cool but
soft and/or rough when at full operating temperature, it is too rich in the midrange and the needle
should be lowered.
- If the engine pulls better when fully warmed up but still not great between
raising the needle to richen 2500-3500 rpm.
- If the engine pulls equally well between 2500-3500 rpm when cooler as compared to fully warmed
up, the needle height is probably properly set.
- Do not pay too much attention to the low-end richness when you are changing needle clip
positions - you still need to be using the clip position that produces the best full
throttle / 5k-7k power in conjunction with the main jets (Step
1) that produce the best power at
high rpm. You will deal with the low-end / cruise next.
Low end (full throttle /
1500 to 2000 rpm)
Step 1 (Best
Main Jet) and Step 2 (needle height) must be selected before starting step
- Float height (AKA fuel level & how to..)
- 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 1500 to 2000 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
- If the engine has a "wet" rhythmic, soggy area at full throttle /
1500 to 2000, 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 / 1500 to 2000 leaner.
- If the engine is "dry" and flat between 1500 to 2000 rpm, raise the fuel level.
- Example: change float height from 15mm to 14mm to richen up that area.
- 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 rpm.
- 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 EXTREMELY IMPORTANT!!!
- 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). See Worn Needle and Worn
Needle Jet diagram. 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! Factory Pro produces stock replacement needle jets / emulsion
tubes for 36mm and 38mm Mikuni carbs. Click
- 4. Idle and low rpm cruise
- Fuel Screw setting (AKA "mixture screws")
- There is usually a machined brass or aluminum cap over the fuel screws on
all but newer Honda. It's about the diameter of a pencil. Cap removal details. Newer Honda carbs
have no caps, but use a special
"D" shaped driver, usually supplied in the carb recal kit.
We do have them available separately, too. 800 869-0497 to order
- Set for smoothest idle and 2nd gear, 2000 rpm, steady state cruise
operation. Set mixture screws at recommended settings, as a starting
point. For smoothest idle, 2nd gear, 2000
steady state cruise , and 1/8 throttle high rpm operation. (pj
- Pilot fuel mixture screw settings, float level (but, you've
"fixed" the fuel level in Step
3 - which you have already done!) AND pilot jet size are the primary
sources of mixture delivery during 2000 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 required.
- 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 mixture.
- 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
- Successful carb tuning is a combination of science, art, intuition and a lot of wizardry.
tuning kits have been thoroughly tested to ensure easy, trouble-free, optimized
- Please note: If you have installed the kit and gone through the optional screw settings,
clip positions and main jets, and still have a persistent flat spot/problem, we ask you to
call us. Unique engine/exhaust/filter/altitude/temperature combinations may require
individualized setups. We are here to help. The information gained to your solution will
be installed in our computerized reference database. PLEASE CALL!
- We ask that upon completion of installation and tuning, that you call us with
specifications of your installation, (pipe brand, filters, advancer, altitude, humidity,
temperature and final carb settings) to be entered in our TUNING DATABASE. The database
allows us to include the "most used" jet sizes and setup specifications in every
- Use (415) 491-5920, (800) 869-0497 or fax (415) 492-8803.
Authorized links to Factory Pro CV Carb Tuning Instructions:
July 20, 2002