Keihin Carburetor Jetting


The air screw is a small (5mm in diameter) slotted brass adjustment screw located on the inlet side (air filter side) of the carburetor. The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The airscrew works with the pilot/slow speed system of the carburetor, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feedback on the necessary setting required for the pilot jet. The airscrew is adjusted in a rather straightforward manner.

The ideal procedure for setting the screw in the correct position is to warm up your scooter engine to the proper operating temperature. Then turn the idle up so it is idling about 500 RPM’s higher than normal. Next turn the airscrew all the way in until it bottoms out, once bottomed out slowly back the screw out a ¼ turn at a time (give the engine 10-15 seconds between each ¼ turn of the screw, to allow the engine to catch up with the adjustments). Continue backing the airscrew out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from 0-1 turns out this means the pilot setting is on the Lean side and a larger pilot jet should be installed. If the engine idles at its highest RPM at over 2 turns out, this means the pilot setting is on the Rich side and a smaller pilot jet should be installed.

If you get no RPM fluctuation when adjusting the air screw there is a very realistic chance that there is something clogging the pilot/slow speed system. Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled.



The pilot jet is a medium size (¾-1”) brass jet located inside the float bowl next to the needle jet/main jet location. The pilot jet meters the fuel required for engine starting, idling and the initial throttle opening 0-⅛.

A lean pilot jet setting will cause your engine to surge at very low RPM’s, bog or cut-out when the throttle is opened quickly and have trouble idling down.

A rich pilot setting will result in hard starting, plug fouling at low RPM’s, sputtering as the throttle is cracked opened.

The pilot jet isn’t difficult to set. With proper air screw adjustment and a close initial setting from your engine tuner, fine-tuning should be painless. Once set the pilot jet is not terribly sensitive. You should only be required to adjust the setting when confronted with large weather changes or altitude swings of over 2000 ft.

If adjusting the pilot jet gives inconsistent feedback, or does unexplainable things. Check and clean out the pilot/slow speed system thoroughly with contact cleaner and blow out with compressed air.



The slide not only monitors how much airflow goes into your engine (its main job). But it has various angles cut on the bottom of the slide to monitor airflow at low RPM’s. This is referred to as slide cut away. The slide cut away is measured in 4.0, 5.0, 6.0 etc. The higher the number, the larger the cut away the leaner the slide setting is.

The slide cut away generally effects the jetting in the ¼ throttle range at almost the same throttle position as the needle diameter effects. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner.

For ¼ throttle jetting adjustments it is easier to adjust the needle diameter.



The jet needle is the most important component in determining your carburetors jetting. The needle is broken into 3 main functions; Diameter, Length, Taper. These needle functions have a large effect on the carburetors jetting from ¼ to ¾ throttle. In the following paragraphs I will explain the needles functions and how to adjust them.

DIAMETER: The needle diameter controls the jetting just above the pilot jet, right as the engine begins to pull. On most engine combinations the needle diameter is felt in the ¼ throttle range. The setting of the needle diameter is crucial to both the engines low RPM power and reliability.

The jetting at ¼ throttle is adjusted by changing the diameter of the needle. On gold colored needles identified with the 3 stamped in letter I.D. system the last letter refers to the needle diameter size. In many instances you can leave the taper and length settings the same (if they are correct) and adjust only the diameter. EXAMPLE: If you have a needle marked DGJ and change it for a needle marked DGK, you have effectively Leaned the jetting at the ¼ throttle position.

When the needle diameter is Lean the scooter will have a loss of low-end power. The engine will feel very zingy. When an engine is in this condition and then put under a heavy load the engine becomes very susceptible to seizing.

When the needle diameter is Rich the machine will sputter at ¼ throttle and be hesitant to take the throttle. In extreme cases the engine can feel like the choke is on or the plug is fouling.

When the needle diameter jetting is set correctly the engine will accelerate evenly thru the first part of the power band. The proper diameter setting will provide maximum low RPM power and very ride able throttle response.

It is important to remember that even though the needle diameter is mainly responsible for the jetting at ¼ throttle there is some bleed effect. With experience this can easily be deciphered. An excellent way to pin point the feel of the needle diameter is test needles in your machine that have both the same taper and length but richer and leaner diameter settings. Try a needle of each setting in your machine for 10-15 minutes of riding and you will begin to understand specifically what throttle position you’re dealing with.

LENGTH: The needle length is determined by the clip position (grooves at top of needle) setting on the upper portion of the needle. On most needles there are 5 clip positions. The top clip position is referred to as #1 and is the Leanest setting. The clips are referred to in numerical order with the bottom position being #5, the Richest. The clip/length setting covers the largest percentage of jetting in your carburetor. With an emphasis at ½ throttle, the clip (length) setting will bleed both up and down to some degree to cover a wide portion of the midrange jetting.

When the clip/length setting is Lean the machine will be very zingy sounding . Lean in the midrange will also rob power and cause the machine to run hot and seize easily

When the clip/length setting is Rich the scooter will have a lazy feeling in the midrange. Exhaust note will be a little flat sounding. In extreme cases of richness the engine will even sputter or kind of crap out in the midrange.

The safest way to set the clip position is to richen up the clip position setting until the machine loses a little power (feels lazy/unresponsive) then lean it back one position. Ideally you like to run the needle setting in either the 3rd or 4th clip position, if possible. The needle clip jetting is especially critical to your scooters reliability because on average more time is spent in the midrange than any other part of the throttle. Most machines pull very hard in the midrange, putting quite a load on the engine. This makes a lean condition very detrimental to your reliability.

TAPER: The needle taper is the angle of the needle at its lower half. The taper works the transition between the midrange and full throttle/main jet (¾ throttle position). The taper is the least sensitive function of the needle. Changes in the taper have very mild subtle changes in the jetting. The taper also affects the main jet size your carburetor requires. A leaner needle taper will use a richer main jet than a comparable engine/carburetor combo with a richer needle taper.

As a general rule, your engine tuner or engine manufacture should preset the taper. Once set correctly by a professional the taper setting should not need to be changed except for cases of extreme temperature reduction.



The main jet affects the jetting in the upper quarter of the throttle position. Coming into play at ¾ throttle on through to wide open throttle. Even though most people relate the main jet to their carburetor in general, the main jet is only responsible for the last ¼ of the jetting. The main jet does not affect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your scooter’s overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.

When the main jet is Lean the engine will experience detonation or “pinging”. Exhaust note will be of a higher, tinnier type note. Engine will over heat easily and can be down on horsepower. A moderately lean main jet can cause engine seizures. A severely lean main jet can cause the engine to burn a piston (whole in top).

When the main jet is Rich the engine will be a bit flat or lazy at ¾ to full throttle, giving off a flat, dead sounding exhaust note. When the main jet is severely rich the engine will sputter in the high RPMs and have a lot of trouble making power up top.

The safest way to get the main jet setting as near correct as possible is to richen the main jet setting up until the engine begins to lose power and not rev to as high of RPM as before.

As a general rule, richen the jetting up as long as the engine likes it and continues to run just as well or better than the smaller size main jet previously installed. When the engine no longer continues to improve its performance you will know you have gone too far.


JETTING: Getting the most out of your scooter

For whatever reason it becomes necessary to re-jet a carburetor, it is without a doubt a nightmare if you do not have a procedure to follow. The following is nothing more than a technique, procedure, steps or whatever you want to call it to help identify and isolate the carburetor circuit involved.



This is the most important step in jetting your carburetor!

Remove the main jet.

Place needle clip in mid-position.

Start engine and run it on the stand.

Condition: engine running and main jet out. Needle or needle jet is correct: Carburetor should run clean to approximately ¾ throttle. From ¾ throttle to full throttle, the engine should start to break up as a result of too rich condition.

Correction: None needed.

Condition: Needle or needle jet is too rich. Carburetor runs clean to approximately ½ throttle but breaks up before ¾ throttle as a result of too rich condition.

Correction: replace needle with next leaner diameter and test again.

Condition: Needle or needle jet is too lean: Carburetor runs clean beyond ¾ throttle and has an erratic throttle response.

Correction: replace needle with next richer diameter and test again.

The emphasis here is to find the correct needle or needle jet diameter, which will allow more fuel to pass than is needed but not so much that the needle itself has no control below ¾ throttle.



Make sure the scooter is warmed up if at all possible.

Main jet out.

Needle clip in mid position.

Turn air screw all the way in then ¼ turn out.

Start scooter and run it on the stand.

Adjust idle so the scooter will just barely idle.

Condition: scooter running and main jet out.


PILOT JET CORRECT: With one hand on the throttle maintaining RPM at approximately ⅛ throttle, turn air screw ¼ turn at a time clock wise until you bottom it out. engine should become slightly erratic and you should have to play with throttle to maintain RPM. Start turning air screw counter clock wise, ¼ turn at a time until you have reached 2 ¾ turns out. Between 1 ¼ and 2 ¼ turns, your engine should have reached its highest RPM maintaining a steady throttle. Adjust air screw again between 1 ¼ and 2 ¼ until you have determined highest RPM. Quick throttle response should be clean without bog.


RPM does not reach a peak between 1 ¼ and 2 ¼ turns, stays the same or keeps rising out to 2 ¾ turns.

Correction: replace pilot jet with next leaner and test again.


RPM does not become erratic and engine maintains throttle when air screw is turned all the way clockwise.

Correction: Keihin replace pilot jet with next richer and test again. Remember, with a steady throttle approximately ⅛, there should be a distinct difference in RPM from 1 ¼ turns to 2 ¼ turns if the pilot jet is correct. The emphasis here is to find a pilot jet that will run crisp without bog and without the main jet.



The main jet selection process is easy once you have the correct needle diameter or needle jet. You now only have to correct a rich condition from ¾ throttle on up and you know what a rich condition sounds like. Your pilot circuit is correct and without bog.

Replace main jet with one that is at least two sizes smaller.

Needle clip in mid position.

Start scooter and run it on the stand.

By replacing the main jet with one that is too small, you are looking for a condition that is too lean. You adjust your main jet from a too small to lean condition.

Condition: scooter running and main jet in.


Carburetor should run clean and crisp to full throttle.

Correction: None needed.


RPM reaches a peak slowly with a deep sound. Excess fuel and oil mixture at end of silencer. Spark plug fowls easily and is dark in color.

Correction: replace main jet with next leaner and test again.


RPM reaches a peak quickly but erratically. A quick full snap open of throttle causes the engine to hesitate BEWAH sound or a complete bogs. Engine sounds like it has a ring to it. End of silencer white. Spark plug is white in color.

Correction: replace main jet with next richer until the BEWAH bog just barely goes away, then replace the main jet with the next richer and run it. The emphasis here is find a main jet that is just rich enough to allow you snap the throttle wide open without the engine bogging as a result of the main being too lean. Should be a quick crisp throttle with no hesitation.


This step in the jetting process can be made very simple if you remain close to stock. However, your needle taper is adjusted for ½ throttle to ¾ throttle. Start off with a rich taper (shallow taper angle) and keep going leaner (steeper taper angle) until it will not maintain constant RPM at ½ throttle (runs erratic). Go back to the leanest taper angle that ran the smoothest at ½ to ¾ throttle and that should be the correct taper.

The needle taper final test should be under track conditions with the greatest effect entering and exiting corners. Do not change the needle diameter or needle jet size during this process because that has already been determined. Adjust taper and throttle cut away only.

Throttle cut away effects from idle to ¼ throttle. The correct cut away will maintain steady ⅛ throttle with quick throttle response. Generally the stock cut away is very close. Experiment with different cut away until it maintains the best response to ¼ throttle.

That’s it, if you spend the time jetting correctly the benefits you will gain definitely out-weigh way the time spent.



Jetting Tip: pilot jet and air screw

To check to see if your pilot jet and air screw are adjusted right, warm up the scooter and let it idle as low as possible while still running smooth. From idle, whack the throttle wide open then let it close completely. Listen to the engine. If it bogs right when you whack the throttle open, then revs up, turn the air screw in ½ turn then try again. Do this until it revs up crisply. After the engine revs up, listen to it revving back down. If the revs drop quickly, and the engine starts to bog, and/or die, then you’re too rich on the air screw, back it out ¼ turn at a time. If after you let the throttle off the engine tends to run on and on while revving down very slowly, you’re too lean and need to turn the air screw in ¼ turn at a time. You want the revs to come up from idle quickly and smoothly, then drop back down to idle the same way. If you turn the air screw all the way in and it still needs to go further, then you will need a larger pilot jet. The opposite is also true: if you are backing the screw out so far that it falls out, you will need a smaller pilot jet.


Shiny Red Tuning

Unit 24, Merretts Mill Industrial Centre






Tel: +44 (0)1453 298212


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