2 stroke jetting explained
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best.
It's very important that you start with the pilot circuit. The reason is
simple. The pilot circuit affects the entire throttle range. When you
are at full throttle, the main jet is the primary fuel metering device,
but the pilot is still delivering fuel as well, adding to the total
amount of fuel that your engine is receiving.
Before you start to rejet your bike, you need a clean air filter, a
fresh plug (actually you need several plugs to do plug-chop tests for
the main jet), and fresh fuel. One important detail: Make sure the
engine is in good mechanical condition. If your engine has a worn
top-end, fix it first. Trying to jet a worn out engine is a waste of
time. The same goes for reeds that don't seal properly, and a silencer
that needs re-packing.
Before you start the jet testing, Install a fresh plug valve. Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the air screw all
the way in, then turn it out 1.5 turns to start. Start the engine, and
turn the idle screw in until you get a slightly fast idle, or hold the
throttle just barely cracked, to keep the engine idling. Turn the
airscrew slowly in, and then out, until you find the point where the
idle is fastest. Stop there. Do not open the screw any farther, or your
throttle response will be flat and mushy, and the bike may even bog.
This is only the starting point, we will still have to tune the air
screw for the best response.
Now is the time to determine if you have the correct pilot installed in
your carb. The air screw position determines this for you, making it
very simple. If your air screw is less than 1 turn from closed, you need
a larger pilot jet. If it is more than 2.5 turns from closed, you need a
smaller pilot jet.
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