Carburetor Tuning

[Texas Pete]

JETTING – Getting the most out of your bike
By Don K. Courtney

To summarise various parts of the carb work as follows:

• From cold, closed to 1/4 throttle, choke and idle jet.
• When started, closed to 1/8 throttle idle jet and air screw.
• 1/8 to 1/4 throttle, slide cutaway.
• 1/4 to 3/4 throttle, needle jet (atomiser) needle position and taper of needle.
• 3/4 to full throttle, main jet and power jet if used. Note: The power jet only works when the engine is revving flat. All it does is allows a smaller main jet to be used which weakens off low down carburation. The power jet substitutes the main jet size when the engine is revving high. Both main and power jet should add together to a normal main jet size.

Below is a more comprehensive and definitive guide to jetting your carb.

For whatever reason it becomes necessary to re-jet a carburettor, 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 carburettor circuit involved.

You can only begin re-jetting your carburettor if the following conditions are met:
1. Top end is in good condition.
2. Bottom end is in good condition. Crank seals.
3. Spark plugs, air filters, reeds and so on.

If your bike’s motor is not mechanically sound, then all the jetting in the world will not help. With all of the above conditions met, you should be able to jet your carburettor following these steps:

Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.

Whether or not your carburettor is a MIKUNI or a KEIHIN, it does not matter. This is the most important step in jetting your carburettor, period!
1. Remove the main jet.
2. Place needle clip in mid-position.
3. Start motor and run it on the stand.

Condition: Motor running and main jet out.

Needle or needle jet is correct: Carburettor should run clean to approximately 3/4 throttle. From 3/4 throttle to full throttle, the motor should start to break up as a result of too rich condition.
Correction: None needed.

Needle or needle jet is too rich: Carburettor runs clean to approximately 1/2 throttle but breaks up before 3/4 throttle as a result of too rich condition.
Correction: Mikuni replace needle jet with next leaner and test again. Keihin replace needle with next leaner diameter and test again.

Needle or needle jet is too lean: Carburettor runs clean beyond 3/4 throttle and has an erratic throttle response.
Correction: Mikuni replace needle jet with next richer and test again. Keihin 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 3/4 throttle.

Step 2: DETERMINE THE CORRECT PILOT JET.

1. Make sure the bike is warmed up if at all possible.
2. Main jet out.
3. Needle clip in mid position.
4. Turn air screw all the way in then 1/4 turn out.
5. Start motor and run it on the stand.
6. Adjust idle so the bike will just barely idle.

Condition: Motor running and main jet out.

Pilot jet correct: With one hand on the throttle maintaining RPM at approximately 1/8 throttle, turn air screw 1/4 turn at a time clock wise until you bottom it out. Motor should become slightly erratic and you should have to play with throttle to maintain RPM. Start turning air screw counter clock wise, 1/4 turn at a time until you have reached 2 3/4 turns out. Between 1 1/4 and 2 1/4 turns, your motor should have reached its highest RPM maintaining a steady throttle. Adjust air screw again between 1 1/4 and 2 1/4 until you have determined highest RPM. Quick throttle response should be clean without bog.

Pilot jet too rich: RPM does not reach a peak between 1 1/4 and 2 1/4 turns, stays the same or keeps rising out to 2 3/4 turns. Correction: Mikuni replace pilot jet with next leaner and test again. Keihin replace pilot jet with next leaner and test again.

Pilot jet too lean: RPM does not become erratic and motor maintains throttle when air screw is turned all the way clockwise. Correction: Mikuni replace pilot jet with next richer and test again. Keihin replace pilot jet with next richer and test again. Remember, with a steady throttle approximately 1/8, there should be a distinct difference in RPM from 1 1/4 turns to 2 1/4 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.

Step 3: DETERMINE THE CORRECT 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 3/4 throttle on up and you know what a rich condition sounds like. Your pilot circuit is correct and without bog.
1. Replace main jet with one that is at least two sizes smaller.
2. Needle clip in mid position.
3. Start motor 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: Motor running and main jet in.

Main jet correct: Carburettor should run clean and crisp to full throttle. Correction: None needed.

Main jet too rich: 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 colour. Correction: Mikuni replace main jet with next leaner and test again. Keihin replace main jet with next leaner and test again.

Main jet too lean: RPM reaches a peak quickly but erratic. A quick full snap open of throttle causes the motor to hesitate BEWAH sound or a complete bog. Motor sounds like it has a ring to it. End of silencer white. Spark plug is white in colour. Correction: Mikuni 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. Keihin 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 motor bogging as a result of the main being too lean. Should be a quick crisp throttle with no hesitation.

Step 4: DETERMINE THE CORRECT NEEDLE TAPER AND CUT AWAY.

This step in the jetting process can be made very simple if you remain close to stock. However, your needle taper is adjusted for 1/2 throttle to 3/4 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 1/2 throttle (runs erratic). Go back to the leanest taper angle that ran the smoothest at 1/2 to 3/4 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 1/4 throttle. The correct cut away will maintain steady 1/8 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 1/4 throttle.

Quick tips: Keep it simple, buy the optional OEM needles or needle jets that are available, as this may speed up the taper selection process. Don’t skip any steps or you’re just guessing.

Final thoughts: That’s it, if you spend the time jetting correctly, the benefits you will gain definitely out-weigh way the time spent. Take the guesswork out jetting by following a procedure that has been given or one you have laid out yourself. Keep the black magic process out of your tuning tricks and you will be better off for it.