Have a website? Want to provide tuning information?
Please do not plagiarize this page - it took years to develop and write and
is not based on any other previous work by anybody else. If you want to link
to this page on your site, please ask - we are perfectly willing to allow
links as long as proper credit is provided. Thanks - Marc
Follow steps in order...., dial in:
1. Top end (full
throttle / 7.5k 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 7500
rpm), select the main jet that produces the hardest pull 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 /5k-7k)
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 / 5k-7k 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/5k-7k
in a full throttle roll-on starting at <3k 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 5k-7k, try
raising the needle to richen 5k-7k.
- If the engine pulls equally well between 5k-7k 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 / 2k-3k)
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 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
- 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 leaner.
- If the engine is "dry" and flat between 2k to 3k 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, 4k rpm, steady state cruise operation. Set mixture
screws at recommended settings, as a starting point. For smoothest idle, 2nd gear 4000
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 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 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
- Carb Kit Design is a combination of science, art, intuition and and
at times, a fair dose of wizardry. There is no dyno that "tells" one how
to assemble or modify the carb to deliver proper power and response.
- Perfect Carb Kit TUNING requires patience and perseverance
and "reasonable" feel to feel the changes - of which - most motorcycle
riders have a good ability to do.
When a dyno "operator" says he/she has to ride the bike after dyno
tuning to do the final tune for cruise smoothness - that's what they are
doing. Avoid any dyno operator who says that they don't have to do
The only dyno that I know of that will duplicate and visually display
the engine smoothness is the EC997
dyno (yes, I know, we make it) - that's one reason why, if you can, you'd
like to use one for tuning - a smoother engine IS getting the best
mixture. Other dynos claim to "tune to an "A/F Ratio" - probably the
biggest marketing scheme in the dyno industry at this time - and they
never can equal the quality of tune as designed -
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.
Marc W. Salvisberg
Authorized links to Factory Pro CV Carb Tuning Instructions:
July 20, 2002