|
(from www.yellowdogracing.com)
So your interested in jetting, eh? Sit back and I'll tell you how to go about
doing it.
Let's start with the basics, and learn how to change a Main Jet. It's very
easy to do.
You need a 17mm wrench and a 6mm wrench or a screw driver with bits that you can
change.
Loosen the carb in the clamps, rotate the carb so you can see the large nut on
the bottom of the float bowl. Use the 17mm wrench to remove the nut (right
tight, left loose) and under this nut, you'll see the main jet. Take the bit out
of the screwdriver, slip the screw driver over the jet and unscrew it (counter
clockwise). You now have the main jet out. Easy, huh? When you put it back in DO
NOT over tighten it. Just snug it - no macho man crap, just snug.
On the side or on the bottom of the jet you'll find a very small number
inscribed in it. This is the jet size. The larger the number, the larger the
jet. The more fuel it will flow and the richer the carb will be.
Okay, let's get down to the tuning aspect of your carb. You'll want to do
this each time you replace a part - especially if you put on a
"performance" part. (new air filter, air filter setup, pipe, etc.)
Start keeping a log of your tuning. WRITE IT DOWN in your manual, or a
notebook or some place that you'll be able to go back and find.
Write down the following stuff:
Main jet size
What position the clip on the needle is in (top groove is #1, bottom groove is
#5)
Screw in your air screw until is just quits moving. DO NOT TRY TO TIGHTEN IT,
just until it stops and count the number of turns. Do this SLOWLY and pay
attention. Keep track in 1/8 increments. Write this number of turns down too.
Most of the time this is somewhere between 1-1/2 to 1-3/4 turns
There's also a jet called a Pilot jet, but you have to take the four phillips
head screws out of the bottom of the carb and pull off the float bowl to find
it. (to remove the float bowl, make sure you use a screwdriver that fits into
the heads of the screws snugly - they call them butter bolts because they'll
strip just like they were made of butter - pain in the ass). The pilot jet will
have a number on it too. Anywhere from 25 to 40, depending on the carb and
application.
It is very important that you do this. (write stuff down) It gives you a base
line to always go back to if you need to.
You should have a little section in the book now that says something like the
following:
September 28, 2002
Main jet - 150
Needle - #3 groove
Pilot Jet - #35 (this is not as important right now, so don't worry about it too
much)
Air Screw - 1-3/4 out
Plug - BR8ES
Let's start on the main jet:
The main jet only works on the upper end of the throttle - approximately 3/4ths
to wide open.
Take your ride out and get it warmed up.
Run it full throttle, get it going as fast as it will go for a block or two and
then hit the kill switch, keep the throttle wide open, put it in neutral and
roll to a stop. - still have the throttle wide open.
Pull out the plug.
What does your spark plug look like after a wide open throttle
(WFO) run?
What color is the electrode? (the part that has the little tit sticking out of
it under the little bar)
Is it black? If so, then it's rich. Drop jet size ONE
Is it white? Then it's lean. Raise jet size ONE
It should be about the color of a pancake or waffle.
For now, change jet sizes in ONE STEP increments. (one size is not 150 to
151, it will jump say to a 152 or 153 - same the other way - prolly a 147 or
something near that)
GO BACK AND RECORD YOUR NEW MAIN JET NUMBER
------------------
Now, let's go look at the bottom end of the carb tuning.
Get your ride idling in neutral (warm it up fully first)
Quickly nail the throttle
What does the engine do?
Does it hesitate and then rev up?
As it comes back down to idle, does it seem to hang at a higher idle for a
minute and then settle down to normal idle?
If it does this, you're too lean on the air screw adjustment. You need to
richen it up a touch. Sorry, I don't know which way to tell you to turn the
screw - that depends on the carb. Use the manual.
Or
Does it sound like it bogs a bit?
As it comes back down to idle, does it seem to drop to a low idle (or even die)
and then come back to normal?
If it does this, it's too rich. Once again, refer to the manual for adjusting
the air screw to lean it out.
ADJUST THE AIR SCREW IN 1/2 TURN INCREMENTS AT FIRST, then fine tune
with 1/4 turns.
Repeat the adjustment until it responds without hesitation and drops back to
idle without hanging or falling and then coming back up. Expect to do the bottom
end adjustment at least three or times before you get it right.
GO WRITE DOWN HOW MANY TURNS OUT YOU ARE NOW!!!!!!
Now that you have a base line setting, try adjusting the air screw a full
turn either way and see how the engine reacts. You'll soon see what the air
screw does. Since you WROTE DOWN what you found, you'll be able to go
right back to that adjustment point.
--------------------
Now that we've done that, what was the overall tuning direction of what you
did?
Did you end up leaning out the upper end and the bottom end? If so, chances
are very good that it's rich in the mid range too. (and vice versa if you were
lean everywhere)
If you leaned out on top and richened on bottom (or richened on top and
leaned out on bottom), chances are that you won't need to play with the needle's
settings.
This is where the needle comes into jetting.
The needle is inside the slide. You have to remove the cap that the throttle
cable comes into. It just unscrews, pull it out and a large spring and a slide
with the needle will come out.
The needle is removed by pulling the spring out of the slide body. You should
be able to see it now. T here might be a retaining clip or washer on top of it.
(once again, this depends on the carb - go use that manual!)
Now, this is a point that confuses people a lot (see, a - lot, not
alot).
To lean out a needle, you move the needle DOWN by moving the little
c-clip UP.
You richen the needle by moving it UP - done by moving the clip DOWN.
You'll hear people talking lowering the needle by raising the clip (leaning) or
raising the needle by lowering the clip (richen). You just have to stop and
think about what was said.
Get the vehicle rolling, about 1/4 throttle and then feed in the throttle
(you want to take about 1-2 seconds feeding in throttle - quickly, but not just
snap it open)
What does it do?
Does it come off the bottom end well and then seem like it gets sluggish?
Then you're too rich - drop the needle by raising the clip ONE position.
Does it seem like it wants to pull through, but just doesn't have any go?
Then it's lean.
Raise the needle by lowering the clip ONE position.
GO WRITE DOWN WHERE YOUR CLIP IS!!!
------------------------------------
Depending on how out of range your jetting is, you may have to work with the
needle, air screw and main jet a couple times to get it right. Don't be
discouraged if it doesn't work the first time - learn from it instead.
Back for more, eh?
Here's a great article (that I stole from dhjunkie) on what the parts of
the carb are and what they do:
This is from R&D Aerosports page and thought it might help those about
what part does what in the carb. (Keep in mind, this was written about a
particular carb - the sizes listed are not the only available - see local dealer
for more)
What Jet Does What?
The Pilot System: The pilot or idler jet controls the air/fuel mixture
mainly from closed to 1/4 throttle. Idler jet comes in sizes 35 to 60 at a 5
unit increments.
The Throttle Valve: The round, flat, or oval cylinder (slide) that
rides inside the main carb body is the throttle valve. This part is rarely
changed in tuning and the same style is used on all Rotax engines. These valves
do come in different cut-away configurations which changes the angle of the
diagonally cut surface, but they are expensive to change and hard to find. If
you need a leaner mixture you can file down the first taper, which increases the
angle of the cut-away. This will lean out the mixture.
The Needle Jet Circuit: This circuit is actually two toning pieces
working in conjunction with each other.
The Needle Jet is the cylindrical brass passage located directly above
the main jet. This part is available in sizes 268, 270, 272, 274, 276, 278, and
280. The smaller the number, the smaller the diameter of the inner passage and
the leaner the condition. The jet needle is the pin that rides inside the
throttle valve and out the bottom of this slide into the needle jet passage.
The Jet Needle: This part is available in 6H2, 802, 8G2, 8L2, 15K2, 11G2,
11K2, 15K2, and 15E5U. There may be a lot more available, but it's a well-kept
secret, at least in this sport. A general rule of thumb to apply is, needles
with a "High Number Code" produce richer mixtures above half-throttle.
Example: 8L2 instead of 6L2. Needles with a "High Letter Code" produce
richer mixtures below half-throttle. Example: 6P2 instead of 6D2. My
recommendation is if you have a mid-range throttle problem, change the needle
jet because this part will consistently affect the entire range from 1/4 to 3/4.
Or you may want to experiment with your jet needle position.
The Jet Needle Position: This position is determined by which location
the holding plate is installed on the jet needle. This position affects the
timing of the jet needle versus the throttle side and the needle jet. This
relationship requires some thinking. It may require you to read this section
several times, but once you understand what's going on, you can make educated
changes rather than random experiments.
Moving the holding clip to the top notch (position #1) will delay the timing
between the jet needle and needle jet clearance versus the air intake supplied
by the opening of the throttle valve. This will produce a leaner 1/4 to 3/4
throttle opening, because there will be more air passing into the engine than
fuel allowed. Moving the holding clip to position #4 will accelerate the timing
between the jet needle and the jet clearance versus the opening of the throttle
valve. This will produce a richer mixture from 1/4 to 3/4-throttle opening.
Don't expect miracles! While this change will make a difference, it may not cure
your problem.
A common complaint is that an engine will not operate at a given rpm. The
engine will either drop or gain rpm without a throttle change. This gap can be
as much as 1,000 rpm's and can often occur in the cruise range. Very annoying! A
lot of people attach the holding plate position and then, in frustration, go
after the needle jet/jet needle circuit. This condition, more often than not, is
a result of the tuned exhaust system not wanting to hold that rpm. There is a
chance that a mid-range modification may take care of this problem, but in many
cases, you are stuck with it.
The Main Jet: This part is easy to understand. It is simply a hole of
an exacting size drilled in a hex-sided piece of brass. This passage controls
all the fuel leaving the float bowl with the exception of the bypass circuits
discussed previously. The main jet is available from 125 to 220 in 5-unit
increments, with a few mid-sizes available, such as 146, 158, and 162. The
smaller the number, the smaller the opening, and the leaner the condition. This
part controls the fuel mixture mainly from 3/4 to full range. The majority of
engine seizures and meltdowns occur in this 3/4 to full range. Leaning or
richening the main jet will have a small effect on mid-range jetting.
EGT: CAUTION! Most EGT gauges read slow, it is possible to seize an engine
with exhaust temperatures as low as 800 degrees. It is advised to start with a
rich mixture and only jet down if the exhaust temperatures are low after an
extended period of time. We recommend 1050 degrees for climb out and not over
1150 degrees at cruise.
|
