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I
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Fuel mixture
- Reducing the amount of oil in the fuel will cause a richer condition, but engine damage may result if not enough oil
is used. (less oil, more fuel = richer jetting) The opposite is also true, more oil will tend to cause a lean condition.
Humidity
- The more moisture in the air, the less air available. High humidity may cause a rich condition.
Load
- Heavy loads will require richer jetting, such as racing in deep sand or deep mud, heavier riders and long high-speed sections
of track.
This list of variables is why jetting can vary at the same track from day to day, or even from hour to hour.
Jetting
- If the jetting variables tend to make the motor run richer, you may need to install leaner jets and adjust the fuel metering
screw, needle, and needle position for a leaner condition. If the jetting variables tend to make the motor run leaner, you may have to
install richer jets and adjust the fuel metering screw, needle, and needle position.
The carburetor has several circuits which
control the fuel/air mixture at different throt-
tle openings. It is not the rpm of the engine
but the throttle opening that is significant
when diagnosing problems. At idle speeds to
1
⁄
8
throttle the fuel metering screw and pilot
jet determine the jetting. From
1
⁄
8
to
1
⁄
4
throttle
the straight section of the needle has an effect
in addition to the fuel metering screw, pilot
jet and slide shape. From
1
⁄
4
to
1
⁄
2
throttle the
slide shape and the taper of the needle deter-
mine the jetting. From
1
⁄
2
to full throttle, the
needle and main jet control jetting. The nee-
dle has less effect as the throttle position
nears WOT (wide open throttle) and the main
jet has more. See illustration III-42.
These circuits overlap to a large degree,
but by testing each range of throttle positions
you can begin to determine the proper jets to adjust. For instance, if the bike “bogs” from idle to
1
⁄
8
throttle, you will start by adjust-
ing the fuel metering screw and possibly change to another pilot jet. If the bike seems to be running really rich and smokes alot from
1
⁄
2
to full throttle, you will probably try lowering the needle to lean the midrange, then changing to a leaner mainjet and/or a leaner
needle if required.
Note:
“Bogging” or “bogs” is a condition where you twist the throttle and the engine just sort of dies, until you release the throttle.
This usually indicates an overly rich condition. When the bike is running too lean, it may “ping” or idle high with a “ying—ying—
ying” sound. Cracking the choke will improve a lean condition, but make a rich one worse.
Testing
- To test the different throttle positions, you will have to put a piece of tape on the throttle housing by the grip. Make a mark
on the grip and mark the tape at the closed position. Twist the grip fully open and mark the position on the tape. Find and mark the
center of the tape between your two marks to find the
1
⁄
2
throttle opening. Measure between the first two marks to find
1
⁄
4
throttle, and
then again to find
1
⁄
8
throttle. Find a flat open area where you can ride in a circle. With the bike warmed up, ride in a circle very
slowly and then accelerate to the
1
⁄
8
throttle opening making note of the bike’s response. If it hesitates and bogs, try again this time
cracking the choke open slightly and see if the response improves with a richer mixture. Now go around the circle at
1
⁄
8
throttle and
then accelerate to
1
⁄
4
throttle. Make note of the bikes response. Try the choke if necessary to see if a richer mixture seems to help.
Repeat the process from
1
⁄
4
to
1
⁄
2
throttle, and again from
1
⁄
2
to full throttle. It is quite likely that jetting problems occurs in one or two
of the throttle ranges but not all. Based on your findings, try to determine which circuits seem to be too rich or too lean. Make a
small change to the circuits involved (1 size on the jets, 1 needle position or
1
⁄
4
turn on the fuel metering screw, etc.) and retest. If the
problem goes away or is improved, you are on the right track. If the problem seems worse, make a change in the opposite direction
of your starting point. Because the clutch does not engage until 5-6000 rpm, testing low throttle openings may be difficult with this
method. The bike should rev freely when the throttle is twisted, and not bog or hesitate when the clutch starts to engage. Adjust fuel
metering screw and pilot jet until the throttle can be twisted rapidly to launch the bike without hesitation.
Keep notes of your changes so you don’t become confused and can return to your starting point if you run into problems. Save
your notes for future reference. Include the temperature and any other variables, so that in the future you will have a head start on
jetting for similar conditions. When you get bike to run well, note how the bike responded when it was lean or rich so you can more
easily recognize the signs in the future.
Note that it is much safer to run a little rich than too lean. A rich condition can foul plugs, but an overly lean condition can cause
extremely high combustion temperatures and detonation which will destroy a piston in short order. If the bike is running very hot
and seems to lose power, it is probably running too lean of a main jet. On the other hand, the bike will produce maximum power
Main Jet
Needle jet -
tapered section
Needle jet -
straight section
Pilot jet & fuel metering screw
1/8
1/4
1/2
3/4
Throttle position
Jet Function v.s. Throttle Position
III
III
.42
.42
