Almost everyone has had problems jetting their ATV's engine.
The reasons are changes in the air, changes due to elevation,
humidity and temperature. All these elements affect the amount
of oxygen taken into the cylinder, while the fuel taken into the
cylinder remains the same. This results in a changing fuel-to-air
mixture that affects the combustion temperatures and the resulting
engine power. Too cool means the jetting is too rich and results
in a loss of power. Too hot means the engine is too lean and
results at first in lots of power until the engine seizes.
So how do you monitor the fuel-to-air mixture?
The old tried-and-true method is to run the engine at half,
three-quarters and full-throttle openings, shut down the
engine with the kill switch, coast to a stop and remove the spark
plug. By examining the color of the spark plug and the deposits
left by unburned fuel, you determine the condition of the gas/oil mixture.
Basically, a normal plug should have a light brown to light tan color with
a dry electrode. If it's a light gray or white color, then the mixture is too lean.
Can I tell you exactly what color your plug should be? No.
The color of the plug depends not only on the fuel mixture, but also on
the type of fuel and type and amount of oil additive.
Different oil additives burn with a different level of cleanness.
Many natural oils are not clean-burning and leave a dark plug,
while the newer and better synthetic oils burn very cleanly.
The other problem with jetting in that pump gas now burns cleaner than
it did a few years ago. This is certainly great for the environment, but
it makes plug reading more difficult.
HOW TO READ A SPARK PLUG
To get a good plug reading, the engine should be run for a half-mile
at the desired throttle and load. At the end of the run, kill the engine
and roll to a stop. Pull the plug and examine the color.
The spark plug reading shows the deposits left by the unburned fuel and does
not directly relate to combustion temperatures. Different types of fuels and oils
result in varying combustion temperatures for the same color of plug.
You must find out from the oil manufacturer or your mechanic what is the
desired plug color for the fuel/oil mixture that you are using.
We have charted some information taken on a stock '89 Honda 250R.
As the jets were reduced in size, the performance increased as expected.
If we continued to decrease the size of the jets, the engine would seize from
a too-lean condition.
A DIFFERENT CONCEPT FOR MONITORING FUEL/AIR MIXTURES
Performance increases as the engine is leaned out, until the limits of the
metals in the engine are exceeded. The exhaust gases that are inside the
cylinder reflect the combustion temperature inside the cylinder. Seizure is
caused by a too-hot (lean) combustion temperature inside the cylinder.
We have charted the Exhaust Gas Temperature (or EGT) using my Optak
tuning device that measures these temperatures accurately. We have
noted the temperature changes with jetting size and the overall performance
after each adjustment.
As you can see, the EGT increases as the engine is leaned out and the
combustion temperature increases. We have manufactured, for four
years, the line of Optak EGT monitors for the two-cycle performance
industry. These gauges have a small 1/8" diameter temperature probe
that mounts in the exhaust pipe to monitor the EGT. Versions are present
for single and dual-cylinder quads. A tachometer is standard on each version.
For night riding, a back light display option is available.
The gauge is designed to handle the abuse and vibration of motorcycles
and quads. Prices start at $239.95.
Basically, the goal is to run the engine as lean as possible, without allowing
the combustion temperatures to exceed the limits of the metals in the engine.
After jetting, always check the engine under all possible loads and throttle settings
(pulling hills, powering out of corners and flat-out in all gears), making sure that
at no time is the combustion temperature above the limits of the metals in the engine.
What about the pilot and quarter throttle settings? Good question. At this throttle
setting the vehicle is slowing for a corner and no power is required. What we
suggest is that you set the jetting as rich as you dare. Whey? To cool the cylinder
and piston. When you leave the corner, you will have a cooler engine that can take
the extra load of pulling out of the corner.
Our EGT readings (see chart) are taken with the probe mounted six to eight inches
from the cylinder. We suggest a different set of eGTs for the different type of
performance riding. When racing, all the possible performance is required and the
tuner takes the time to adjust the jetting before each race.
The serious play rider is out to enjoy the day and is not going to want to be adjusting
his carburetor every time he changes elevation or as the temperature changes during the day.
(Note: The desired EGT is independent of the fuel type-race gas, pump, etc.-or the oil
additive type or ratio. The EGT limits are determined by the effect the combustion heat
has on the metals in the engine, not by the type of fuel mixture. Also note that the EGT
limits are the same for different engine manufacturers and vehicle types-motorcycles,
snowmobiles and Jet Skis-along with quads.)
Important rules: Always adjust the jetting from the rich side.
Only make small jetting adjustments at a time: large ones become confusing unless
you are lean, then make a big adjustment to be sure that you are rich.
A spark plug that is showing less color than desired or has too hot of an EGT reading,
means the combustion temperatures is too high, so increase the size of the main jet,
which increases the fuel flow. If the spark plug color is darker than desired, or the EGT
reading is too low, the combustion temperature is below optimal, decrease the size
of the main jet to decrease the fuel flow and lean the engine out.
On the chart, note that the adjustments can interact. Changing the size of the main jet
can change the midrange performance. We suggest that you only make small
adjustments at a time and check the engine performance and EGt or plug readings at the
half, three-quarters and full throttle settings and under full power and flat out condition.
ADVANCED JETTING FOR THE RACER
As you will see, this jetting technique requires more effort.
This level of adjustment is not possible by reading the spark plug and must be done
with an EGT gauge.
The pilot jet has an effect on all throttle settings. Adjust the size of the pilot jet so the
EGT is within desired temperature range for your type of riding. When the idle EGT
is low, the risk of plug fouling is increase at idle.
Make the same adjustments on clip position and main jet as above.
You should now have an engine with EGT temperatures in the desired range from
idle to full throttle, but some areas could possible need some fine-tuning.
Review the information collected after all adjustments have been made.
Then repeat the process at all throttle settings and loads. Look for any peaks or dips
in the EGT reading. Again, follow the #1 rule: make sure that at no point does the combustion
temperature exceed the limits of the metals in the engine.
Adjust the needle taper at change the shape of the cutaway in the slide.
Not only does the size of the needle matter, but the taper of the needle is important.
Also, the shape of the cutaway in the slide is critical.
The slide controls the airflow with throttle position.
The needle adjusts the fuel flow to throttle position. What is important is the ratio between
At full throttle, the slide is wide open and the needle is completely out of the main jet.
So the shape of the cutaway and taper of the needle have no effects.
The adjustment of the needle taper or slide cutaway will allow for the fine-tuning the
shapes of the EGT with performance and throttle setting.
WARNING SIGNS OF TOO HIGH A COMBUSTION TEMPERATURE
As you adjust the engine to the limits, the first sign of too high a combustion temperature
is on the exhaust port side of the rings and piston. You will start to see erosion here, the
piston will lose the sharp edge and the top edge of the ring will become rounded.
When this happens, you know that the limits of the metals in the engine have been reached
and you should reduce the combustion temperature.
EXCEPTIONS TO THE DESIRED EGT SETTINGS
Engine timing: The desired EGT readings given above are based upon standard engine timing.
If the timing is highly advanced, the combustion gases are kept in the cylinder longer, which
results in more cylinder heating. The desired EGT is now closer to 1000 degrees.
Nickel-impregnated. cylinder walls and ceramic top pistons: Nickelimpregnated cylinder walls
have the toughness of steel, and because the nickel is very thin, the heat conductivity of aluminum
is deep. The ceramic on the top of the piton acts as a heat insulator and protects the aluminum
piston from the combustion temperatures and related EGT temperatures and power.The The Optak brand of EGT is no longer being made. 5-24-01
I think this was out of a Dirt Wheels magazine, I will be adding more on EGT's later on If you have any information you want to addon EGT's just email me
More on EGT's