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 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.
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
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.
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