Re: Oil Injection rejection
To properly set your jet size, you need to know how to read your spark plug. I found this procedure
on Trinity Racing's web site. For our purposes, the plug has three things of interest: the ground
strap, base ring, and porcelain. The ground strap is the closest piece to the piston. The base ring
is at the end of the threads and is what the ground strap is welded to. The porcelain is the ceramic
covering for the center electrode.
1. You set your heat range from the ground strap.
2. You do all the plug readings for jetting from the base ring.
3. You determine detonation and timing issues from the porcelain. The porcelain is for
determining preignition/detonation, not jetting or air/fuel ratios.
Finding the correct plug heat range:
The ground strap is your window to getting this right. If the "color" of the ground strap
"changes" too close to the ground-strap's end, (the end opposite of the base ring), then the
heat-range is "too cold", (heat transfer is to quick to the base ring).
If the "color" of the strap changes near where it is welded to the base ring, then it means that the
plug heat-range is "too-hot", (heat transfer to the base ring is to slow causing the deposits to be
burned off the strap completely) The strap at this point could start working like a "glow plug",
probably resulting in pre-ignition/and/or detonation. The properly set heat-range is when the
"color" is at the half-way point on the strap.
You change the plug's temperature by using a lower or higher plug code. For most plugs, a lower
number means a cooler plug. But it is the opposite for NKG plugs where a lower number is hotter.
For example, an NKG BR8HS is hotter than a BR9HS.
Use the base ring to determine jetting:
The base ring "color" is very close to the color of the piston crown and is used to determine the
jetting. Your looking for the soot color to be a nice light to medium brown. If the color doesn't
go all the way around the base ring (at least one full thread turn on the plug) or the color is
whitish it is way too lean. If the color goes all the way around, but there is a spotting of heavy
dry soot on the top of the color, you are too rich. ( TWO STROKE JETTING WILL BE A
LITTLE WETTER LOOKING AND DARKER THEN A FOUR STROKE )
Detonation / preignition:
The first signs of detonation / preignition will be seen on the porcelain down in the plug, It shows
up as tiny black or shinny specks of aluminum. Also Look very close around the center
electrode where the porcelain intersects, this will appear to be melting between the insulator and
Detonation is caused by the air/fuel mixture exploding rather then burning. This gives off a
sound, (a knock) this sound is the result of a shock wave, this wave disrupts the boundary layer
of cooler gases that cover the internal parts of the combustion chamber. This causes a very
rapid rise in pressure and temperature. The results are holes in the top or sides of the pistons,
blown head gaskets, broken rods, (all bad stuff). This can also shock the rings from there seal
causing oil to form as little spots on the porcelain.
Setting the Jet size:
Carburetors have three basic ranges of operation: Idle, mid, and high. Each range needs to be
set for the overall jetting to be correct. Idle goes from idle to 1/4 throttle. Mid goes from 1/4 to 3/4
throttle, and high is from 3/4 to full throttle. There is overlap, but this is a general layout.
The idle circuit is affected by the slow, or idle jet, and the air bleed screw.
The mid range is affected by the metering rod height and to a smaller extent, the main jet. The
metering rod is attached inside the throttle piston via a circlip. The rod has several rings cut into
it at this end into which the circlip is inserted. Raising the rod (lowering the circlip) richens the
mixture. Lowering the rod, leans the mixture.
High range mixture is controlled by the main jet. A larger jet richens the mixture, and a smaller
jet leans the mixture.
To begin, start the engine and let it warm if possible. On the front side of the carb is a little
screw that is recessed in the carb. This is the idle mixture or air bleed screw. It controls the
amount of air that is mixed with the fuel in the idle circuit. Turning out allows more air, or
leans the mixture. Turning in reduces the amount of air and riches the mixture. It is best to first
turn it all the way in to a light stop being sure to count the number of turns since this is your
starting reference point. When adjusting, turn the screw 1/4 turn at a time and wait to see the
result. If the engine is idling smoothly, leave this alone for now. The proper mixture is achieved
when the engine idles smoothly, and returns to a smooth idle after the throttle is blipped.
When adjusting mid and high range, be sure to run the bike at the desired speed range
then hit the kill switch while holding the throttle in that position until the bike comes to
a stop. This preserves the state of the plug. Allowing the engine to return to idle or riding the
bike back to your work area, will leave the plug coated in some other state.
Install a new plug. I always adjust high end first since this involves the main jet. Run the bike at
full throttle for 5-10 seconds, longer if possible. Plugs coat quickly so you don't have to run it for
long. Remove the plug and look at the base ring. If white, it's too lean and install a larger jet.
If black, or dark brown, the mixture is too rich. Install a smaller jet. Repeat this procedure until
the color is tan.
Now do the same with mid range. If your bike has a throttle limiter, remove the air pipe so you
can see the throttle piston and set the limiter to stop the piston mid way. Run the bike as you
did for the high range and adjust the metering rod accordingly. Once this is set correctly, check
the high range again to be sure nothing has changed.
After the high and mid ranges are set, lets set the idle mixture. Sometimes idling won't show on
the plug unless you let the engine idle a long time. It's easier to listen to the engine and let the
mixture accordingly. There are some conditions that are easy to recognize. Surging is usually
do to the mixture being too lean. Turn the air bleed screw in. If the engine wants to stall after
blipping, then it's too rich and needs to be leaned out. If you can't lean it out enough, then you
need a larger slow jet. If it's always to rich, then you need a smaller slow jet. However, I have found
changing the slow jet to be rare.
I hope this helps.