Diminishing returns: Jets come with
different size holes in them.
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I’m a new convert to the two-stroke
revolution, and I’m extremely weak
on how a carburetor works or how to
jet it. Help?
Correctly jetting your carb is
important, not just because it will
keep your two-stroke from seizing,
but because it will improve horsepower, torque, throttle response and
over-rev. A two-stroke carb’s working
(and changeable) parts are the mainjet, pilot jet, needle and air screw.
The only way to correctly jet your
trusty but rusty two-stroke is through
trial and error. In the old days, bikes
came out of the factories with iffy
jetting. And, even if the factories
had perfect jetting, it wouldn’t
necessarily work with your riding
style, track conditions, altitude,
temperature or barometric pressure.
Jetting is very weather-dependent.
Jetting isn’t as complicated as it
seems, though, because the carb’s
major components are broken down
into sizable bites and are uniquely
Let’s start with the pilot circuit.
The pilot circuit has the smallest jet
in the carb and it affects low-rpm
fuel delivery and starting. It doesn’t
have a shutoff valve, so it has some
effect on every rpm range, but it
typically works when the air going
through the carb throat isn’t moving
fast enough to create enough vac-
uum to pull fuel through the larger
mainjet. Pilot jets come in a variety
of sizes, but the easiest way to tell
whether you need a different pilot jet
is by adjusting the air screw.
To test the pilot jet size, set the
air screw at exactly 1-1/2 turns out.
Then, start the engine and hold it at
a fast idle as you turn the air screw
in and out until the engine idle is at
its fastest. This is the sweet spot for
the air screw.
If your air screw is set at less than
1 turn out from all the way in, you
need a larger pilot jet. If it is more
than 2. 5 turns out, you need a
smaller pilot jet.
Never assume that your air-screw
setting is correct. It should be tested
whenever there is a temperature,
altitude or humidity change.
The mainjet is the primary fuel-metering device for every throttle
setting above where the pilot works.
It works on the vacuum of the air
rushing through the carb throat—
typically from 3/8 to full throttle. As
a rule of thumb, most manufacturers
provide three mainjets with their
bikes. The first mainjet is their best
guess as to what will work for the
majority of riders. The other two
main jets bracket it in size to cover
all contingencies. The only way to
test the mainjet is to ride the bike.
While riding, you are trying to
determine whether the main jet is
too rich or too lean. Here are four
clues to help you analyze your jetting
from the saddle.
(1) Pinging. If you hear a
crackling sound, often referred to as
pinging, while the engine is under a
load, the bike is lean. Pinging doesn’t
always ping. More often than not,
it sounds like TV static that can be
heard when you are accelerating out
of a turn. If your bike pings, stop
and put in a bigger main or raise the
( 2) Pop or bog. If your bike
pops when you back it off or when
transitioning from low to mid, you
are lean. Conversely, if the bike bogs
in this area, the bike is rich.
( 3) High-rpm run. Find a big
area and ride the bike wide open
in third or fourth gear. As the rpm
climb to maximum, roll the throttle
back a little bit. If the bike actually
runs better when you roll the throttle
back than it did when you were at
full throttle, you are rich.
( 4) Plug reading. With a trained
eye, you can read the color of a
two-stroke spark plug to determine
whether it is rich or lean. To get a
plug reading, ride the bike across
a field at full throttle. Once you are
wide open, pull the clutch in, kill the
engine and coast to a stop. Do not
let the bike keep running. Do not
ride back to the pits at low speed.
Do not blip the throttle. Any of these
actions will ruin your wide-open plug
Pull the spark plug and look at the
color of the ceramic insulator. If it is
white or chalky gray, the main is too
lean. If it is dark brown or black, the
main jet is too rich. The color you’re
hoping for is a light brown or tan.
The final step, but perhaps the first
step on a bike that was running fine
but starts to exhibit pinging, popping
or bogging, is to adjust the needle.
Since the needle sticks down into
the tube that the mainjet feeds the
engine through, moving the needle
up or down in the tube changes
how much fuel flows at mid-throttle
settings. You adjust the height of the
needle by moving a C-clip on the top
of the fragile aluminum rod. Moving
the C-clip to the top of its five
grooves will drop the needle down
into the tube, blocking more fuel and
thus making the bike leaner. Moving
the clip to the bottom groove will
raise the height of the needle and
richen the fuel. You have five clip
positions to work with—and they can
make a significant difference.
That is the basic primer on how a
carb and its jetting work. The pilot
jet, air screw, mainjet and needle
work in conjunction to control the
fuel/air mixture, but they are just
metal parts. Finding the right jetting
takes a rider who can feel whether
his bike is rich or lean and who has
the patience to make the proper
changes when the situation calls
for it. Good luck with your new