[h=3]JETTING: Getting the most out of your bike[/h]I have had a lot of questions in reference to my article on carburetor jetting, so I have simplified the procedures to elevate as many questions as possible.
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:
- Top end is in good condition.
- Bottom end is in good condition. Crank seals.
- 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:
[h=3]Step 1: DETERMINE THE CORRECT NEEDLE AND OR NEEDLE JET.[/h]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!
- Remove the main jet.
- Place needle clip in mid-position.
- 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.
[h=3]Step 2: DETERMINE THE CORRECT PILOT JET.[/h]
- Make sure the bike is warmed up if at all possible.
- Main jet out.
- Needle clip in mid position.
- Turn air screw all the way in then 1/4 turn out.
- Start motor and run it on the stand.
- 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 TO 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 TO 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.