Carburetor Tuning Tutorial
Major Tuning Adjustments
Main Jet
This controls how much fuel enters the carburator when the
throttle is ¾ to full open.
Pilot Jet
This controls how much fuel enters the carburator when the
throttle is at idle to 1/8 open
Pilot Air Screw
This controls how much air enters the carburator when the throttle
is at idle to 1/8 open
Jet Needle
This controls how much fuel enters the carburator when the
throttle is 1/8 to ¾ open.
How Carburetor Tuning Works
When a throttle is at idle, the jet needle is completely blocking
the main jet, thus the pilot jet (which is always 100% open) controls
how much fuel enters the carburator. The amount of air that enters
the carburator during idle is adjusted by the pilot air screw. Thus,
to control how well your bike idles you must adjust the pilot jet and
the pilot air screw. As you increase the throttle the jet needle
pulls out of the main jet, allowing fuel to move into the carburator
through this opening as well. This is why the jet needle setting
controls how much fuel is entering the carburetor from about 1/8 open
to ¾ open throttle. Above ¾ throttle the jet needle is
almost completely out of the main jet, and thus the main jet alone
controls how much fuel is entering the carburetor.
Possible Adjustments
Main Jet
You can control the size of this jet by changing it in the
carburetor. A smaller size jet reduces the maximum flow of gas that
can enter the carburetor and a larger size jet allows more gas to
flow. Thus reducing your main jet from 140 to 135 will cause your
bike to run leaner on the top end of the throttle.
Jet Needle
Your jet needle should have several slots in it in which an E or C
retaining clip is fastened. By moving this clip up towards the top
of the needle you cause the needle to sit lower into the main jet,
and thus lean out the mixture by letting less gas in. By moving the
clip down towards the tapered end, you are causing the needle to sit
higher in the main jet, thus richening the mixture by letting more
gas flow in. Remember the jet needle is the major adjustment for the
carburetor from about 1/8 or ¼ throttle to ¾ throttle.
Pilot Jet
You can control the size of this jet just as you can control the
size of the main jet, just by changing it. The smaller the jet, the
leaner your bike will run at idle and the larger the jet the richer
it will run.
Pilot Air Screw
This can be used to fine tune the Pilot Jet. The following
applies for any given pilot jet size. Screw the pilot air screw all
the way in, or out depending which was causes less air to flow into
the carburetor. Then slowly turn the pilot air screw out in ¼
turn increments until you reach the desired fuel to air mixture. If
you have to turn the pilot air screw more than 2 turns, you need to
move to a 1 size smaller pilot jet and repeat this process again.
Q and A Session
Questions: Is this really so cut and dry?
Answer: No, this is a "grey" area, because all
of these factors influence the carburator at the same time. However,
the predominant factor is listed above because it has such an
overwhelming effect that the other factors become almost negligable.
If you are going to try and get every drip of performance out of your
bike, I am over simplifying and you need to learn the physics behind
why all this works as it does.
Links to Other Carburetor Adjustment Sites
Carburetor
101 at Motocross.com
If you have any questions or
comments feel free to email me at dave@cs.colgate.edu
Hope this all helped!
Dave Nellans - October 21st, 2001