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Factory CV Carb Tuning
For High RPM Engines
1. Top end (full throttle / 7.5k to redline) - Select best Main Jet
To get the best, most even top end power (full throttle/after 7500 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.
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) - 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.
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 leaner.
- If the engine is "dry" and flat between 2k to 3k rpm, raise the fuel level. For example, change float height from 15mm to 14mm to richen up that area.
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 importatnt!!!
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) 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 cap over the fuel screws on all but newer Hondas. It's about the diameter of a pencil. Newer Honda carbs have no caps, but use a special "D" shaped driver, usually supplied in the carb recal kit.
- 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.
- 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.
- 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.
- 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!
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.
Article from http://www.factorypro.com/tech/carbtun.html.
