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PLUG CHECK & READING SPARK
PLUGS
The first step is to replace
the old spark plug with a new one. Make sure it is the
proper plug for your machine, and that it is gapped
properly. An improper plug gap will throw off the readings.
If the gap is too small, it will not burn properly. If it is
too large, it will cause added stress on other ignition
components, causing premature failure. You should have fresh
fuel at the proper octane level. Most small engines run best
with 92-94 octane fuels. It is also important that you have
a clean air and fuel filters for the tests.
Plug testing is the easiest
way to help you dial in the jetting of the carburetor after
adding performance parts and for different altitude or
weather changes. The color of the spark plugs is used as an
indicator and lets you know what way to start re-jetting the
carburetor.
There are three basic ranges
that a plug test can be used to check.
3/4-Full
throttle, which is controlled primarily by the main jet
1/2
throttle, which is dominated by the taper angle of the
needle and the clip position.
1/4
throttle, which is mainly, controlled by the strait diameter
of the needle but also the slow circuit.
Testing the Main Jet
Once you have a new plug
installed and gapped properly, start the machine and let it
warm up. Take the machine for a VERY short ride to bring it
up to normal operating temperature. Find an open spot of
road and get it up to top gear. Roll on the throttle and
hold
it wide open; 2 strokes for
3-5 seconds. 4 strokes 10 - 15 seconds. While you are still
wide open you must stop/kill the engine using the kill
switch while pulling in the clutch at the same time, all the
while still holding the throttle wide open. Let the bike
coast to a stop with the clutch still pulled in .Do not let
the clutch out until you come to a complete stop.
**** It is very important
that you do not let the engine slow down or idle at all. You
must do it this way for every test!****
Now you need to remove the
spark plug and check its condition. You want to look and
take note at the white insulator part of the plug only, not
the metal tip or electrode.
The insulator of the spark
plug must be a light grayish-tan to light brown color. If
the insulator is white, it is too lean, meaning the main jet
is too small. If the insulator is black or wet, it is too
rich, meaning the main jet is too big.
The steps for checking the
other ranges are basically the same. If you want to check
the 1/2 throttle range you want to hold it steady at 1/2
throttle in top gear for about 20-30 seconds. The same for
1/4 throttle testing, hold it steady at 1/4 throttle. Pull
in the clutch and shut if off at the same time while holding
the throttle at a steady speed.
BASIC CARBURETION
To be able to properly jet or
re-jet a carburetor you must first know how they work. Most
ATV and off-road motorcycle carburetors work basically the
same way. The two most popular models are the Keihin and
Mikuni.
These carbs have two primary
fuel circuits; the slow/pilot circuit and the main circuit,
which have several parts or components that will effect the
mixture at different throttle positions. There are 4
throttle positions that are used to determine what component
is effecting the mixture. They are idle/off idle, 1/4
throttle, 1/2 throttle and 3/4 to full throttle.
It is important that you
remember that the main fuel circuit is only part of the
entire fuel delivery and any changes you make to the slow
circuit will affect the main circuit as well, because the
slow/pilot circuit is delivering fuel from idle to full
throttle. So you must start with the slow circuit and work
your way up to the main circuit.
The Pilot/Slow Circuit
The pilot/slow circuit has
two major components that affect the mixture. They are the
the pilot/slow jet and the air mixture screw. The pilot/slow
jet has a fixed diameter hole that controls the amount of
fuel being delivered through the slow circuit. The mixture
screw is an adjustment screw to control the amount of air
being mixed with the fuel from the slow jet.
The Main Circuit
This circuit has 4 components
that effect the mixture from about 1/4 throttle to wide
open/full throttle. These parts are the throttle slide
cut-away, jet needle or needle, the needle jet and the main
jet. The throttle cut-away effects the amount of air but can
be left alone. The needle primarily controls the mixture
from 1/4 throttle to 3/4 throttle. The needle moves up and
down inside the needle jet as you open and close the
throttle. The diameter of the needle is what affects the
mixture at 1/4 throttle. The taper of the needle and the
clip position effect the mixture at 1/2 throttle up to 3/4
throttle. At 3/4 to full throttle, the mixture is controlled
mostly by the main jet.
Starting From The Bottom
The first throttle range to
dial in is the idle/off idle range. Any time there is a
weather change or you are riding in a different area you may
need to adjust the mixture screw for a clean and crisp
engine response. The two main settings are the air mixture
screw and the pilot/slow jet. The correct setting is the one
that allows a crisp engine response with the air screw about
1 3/4 turns out. If you need to turn the air screw out more
than 2 1/2 turns, then it is too rich and you need a smaller
pilot jet. If you need only to turn the mixture screw out 1
turn, then it is too lean and you need a larger pilot jet.
1/4 Throttle
The mixture in this range is
mostly controlled by the diameter of the needle. A lean
mixture will cause the engine to hesitate and a rich mixture
will cause it to sputter while keeping it at a steady speed.
If there is hesitation, then a smaller needle is needed. If
it is rich a larger one will need to be installed.
1/2 Throttle
This range is controlled by
the clip position and taper angle of the needle. The correct
clip position is often all you need to dial in this range.
The correct position is the one that will allow the engine
to rev cleanly from 1/4 throttle to 3/4 throttle without
hesitation or sputtering. Raising the needle clip will lean
the mixture and lowering the clip will richen the mixture.
If you find that you are all the
way to the top or all the way
to the bottom positions and the jetting is still off, you
may need a different needle jet (if available) or a needle
with different dimensions.
3/4 to Full Throttle
This range is controlled
mostly by the main jet. A correct main jet will allow the
engine to rev cleanly all the way until it drops off the
power band with out sputtering or hesitation. A lean mixture
will have power in the lower RPM's but will get sluggish
and/or hesitate in the higher RPM's. A rich mixture will rev
ok but sputter in the higher RPM ranges
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