General Carburetor Jetting Guide
If you are having idling issues or poor throttle response,
the first thing to check is your air/fuel mixture. The air/fuel mixture controls
from idle to 1/4 throttle. The best way I have found to check it is to let the
bike warm up to operating temperature. Let the bike idle in neutral and give it
wide open throttle. If the bike has a hesitation your mixture is off. If the
bike bogs down and then revs up fast it will be lean. An up and down or
"rolling" idle is another symptom of a lean mixture. If the bike idles rough and
kind of low, it will be rich. The best place to start with your settings is
about 2 turns out. Tighten the screw lightly until it seats and then back out
and count slowly.
If you get more than 3 full turns out on your air/fuel
mixture, you will need to step up on your pilot jet size. If you get under 1
full turn then you will need to step down one size on your pilot jet.
Next Is Your Needle
Your needle will control ¼ through 3/4 throttle positions.
If your bike is "popping" or "missing" at a constant speed, your needle will
need to be adjusted. If you give your bike full throttle from a constant speed
and experience a hesitation, you are lean on your needle. You will need to lower
the clip on the needle (this raises the needle), allowing for more fuel flow
through your needle tube (dump tube). If the bike is trying to "load up" at a
constant throttle position, your needle setting is to rich. Raise the clip on
your needle (this lowers the needle), which will restrict the amount of fuel
coming through your needle tube.
Your main jet screws into a brass tube in your carburetor
called the needle tube (dump tube). If you run out of adjustment on your needle
then you will need to replace it, although very few times have I seen that this
needed to be replaced. The only time that I have seen it need to be replaced is
when running methanol.
Your main jet controls 3/4 to full throttle. If you are at
wide open throttle and the bike is revving up but not really pulling, you will
be lean. If the bike is slow to rev, then you are rich. The best way to check
your main jet settings is to do a spark plug reading. Run the bike at full
throttle until top speed is reached, then shut it off using the kill switch.
Keep the throttle wide open until you come to a full stop. Pull the plug and
look at the porcelain down in the plug. The all around best color is a light
chocolate color on the plug. If you run the bike at full throttle all the time
you might want to run one step bigger on the main jet, to keep the motor from
burning up. If you run short distances at full throttle (ie. racing) you can run
one step smaller on the main jet for maximum power, but keep the full throttle
Some More Jetting info....
Lets start with elevation.
Higher altitude means less air density. This will require
smaller jets. Usually you would need to go down one main jet size for every 1700
to 2000 feet of elevation you ascend.
For example, if you are at sea level and you are running a
160 main jet, you'd need a 140 jet for 4000 feet. In colder conditions you need
a larger jet as cold air is more dense so the air temperature plays a role.
You'll also lose horsepower the higher you go up in
elevation. You can figure on losing about 3% of your power for every 1000
feet you go up even with re-jetting your ride.
Pilot jets control the low speed and idle of your mix. If
adjusting the mixture screw won't improve the low-end running speed, it's
usually time to get a different pilot jet.
the needle controls the fuel mixture throughout the
midrange. The shape or taper of the needle determines how much fuel goes into
the engine at a given throttle position. Your needle is for fine tuning
once you have the right main and pilot jets figured out.If the machine stutters
or hesitates before it brings on the power, that part of the needle's taper is
usually too small and sometimes the only way to fix it is to get a needle with a
different taper. Finding the right needle can be a real pain so try moving the
clip first, in minor cases this will fix the problem, but with engines that have
been highly bored over and have a higher performance exhaust, you are most
likely going to need a new needle, pilot and main jet.
Main jet comes into play at three-quarters open to full
throttle. We all know a bigger jet means more fuel to the engine and it also can
mean a cooler running engine. While a one size smaller jet can mean more
power for a while, it can also mean burning your engine out by running too lean.
A slightly richer main jet can help your engine last longer by helping it run
cooler and still make good power, so be sure that your main jet is big enough.
After all, you can't ride your machine if the rings are shot or the piston skirt
is broken due to overheating.
***Don't change more than one jet at a time. ***