An Alternative Method For Figuring Out Carburetor Tuning

[90GTVert]

The text below is from an article by Don K. Courtney called "Jetting - Getting The Most Out Of Your Bike". I can't seem to find the original article. I know it's been around since at least 2010 and reposted on numerous forums. Searching for the author doesn't bring anything up for me aside from a facebook page that looks like it's possibly the author; based only on a dirtbike profile pic and being the only person that comes up for that name. Unfortunately there is basically no activity there and they didn't fill out any work related info that could help find if it was in a magazine or a webpage or how it was originally posted. There's a scribd document that contains the same text, however it appears to be screenshots of a Lambretta page where it was reposted.

Since all I can find are reposts, I'm going to do the same here because it's an interesting take on carb tuning. I have corrected a couple of spelling errors and modified things like text sizing and spacing to make it easier to read compared to other reposts. I'd rather link to the original to be as respectful as I can to the author, but I'm currently unable to. If you have the original source, let me know.

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JETTING

Getting The Most Out Of Your Bike

By Don K. Courtney

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.

You can only begin re-jetting your carburetor 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 carburetor following these steps:

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

Whether or not your carburetor is a MIKUNI or a KEIHIN, it does not matter. This is the most important step in jetting your

carburetor--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: Carburetor 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.

Condition: Needle or needle jet is too rich. Carburetor 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.

Condition: Needle or needle jet is too lean: Carburetor 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:

Carburetor 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 color.

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 color.

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

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.