Transcript How to Rejet a Dirt Bike

How to Rejet a Dirt Bike
By: James Harley Jr
What does the Pilot Jet do?
• The pilot jet supplies the engine with
fuel when idling.
• When you twist the throttle on the dirt
bike giving it gas, the accelerator pump
squirts some fuel. As the vacuum
slides open, fuel begins to suck
through the main jet and mix with fresh
air coming in, then you go back to zero
throttle. Fuel is then coming through
your pilot jet (only enough to make it
idle).
Why do you Rejet a Dirt Bike?
• You rejet a dirt bike because the dirt
bike is running rich or lean. Running
rich means it is getting to much gas
and running lean means it is not getting
enough. This will make it run better, if
the dirt bike is running to lean the
motor will blow up. If it is running to
rich the dirt bike will lose power.
Different types of Pilot Jets
• Pilot jet- idle jet
• Needle jet- In the barrel of your carburetor.
From 1/4 throttle to mid throttle.
• Main jet- wide open throttle.
The Tear Down to the Pilot Jet
• Step one: take the seat and number
plates off of the bike.
• Step two: take off the air box and air
filter.
• Step three: take off the clamp to the
carburetor, because the pilot jet is in
the barrel of the carbueretor.
• Step four: screw the jet out of the
carburetor
Putting the New Pilot Jet in
• As you were taking the old jet out, and it
was 3/4 in, or 2 1/2 turns out, you need to
replace your jet.
• When you put your new jet in, put it in at 1
1/4 turn and that should be the right place
for the jet to be at,.
Weather
Temperature can play a huge role in how
your dirt bike engine runs. As the air
temperature rises, the air density becomes
lower, causing a richer mixture. To combat
the rich condition, you will have to use jet
sizes with lower numbers to compensate for
lower air density. If barometric pressure
decreases, the opposite effect will occur, and
you will have to adjust for higher air density
with higher jet sizes.
How do I know if a jet is GENUINE MIKUNIE,
GENUINE KEIHIN, or generic? What does this
mean?
• The OEM equips a mikuni carb with GENUINE
MIKUNI jets and a keihin carb with GENUINE
KEIHIN jets. Many providers make generic jets
that may not match the genuine jet.
GENUINE MIKUNE jets
will have stamping on
them which is a "square
within a square" on them
GENUINE KEIHIN
jets will have a "k
star" stamping on
them
What do the Numbers Mean on the
jets?
• The bigger the number is on the jet, the
more fuel flows. This also means the bike
will run richer.
• The lower the number is on the jet, the
less fuel flows. This also means the bike
will run leaner.
What the pilot jet looks like
How the Bike Runs After Being
Rejetted
• After the bike is rejetted it will not run to
rich or to lean. The bike will run perfect
and will idle without stalling. The bike won't
blow out clouds of white smoke from the
exhaust. The bike won't hesitate/miss a
beat.