Jetting Guide: The Yamaha / Keihin FCR Carburetor
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The FCR carburetor consists of 3 separate yet overlapping circuits that control the
air-fuel mixture at different throttle openings (Table 1 and Figure 1). A circuit is the
(emulsifier) air and gas path used to supply the air/fuel mixture at different
throttle positions. The idle or pilot circuit
consists of the pilot (fuel) screw (PS), the pilot air jet (PAJ), and the pilot jet (PJ). The pilot circuit
controls the mixture at idle and up to about ¼ throttle. The needle circuit consists of the throttle
valve, jet needle and the needle jet. This circuit controls the mixture from
~1/8 throttle to 3/4 throttle. The throttle valve cutaway controls from ~1/8 to
1/2 throttle and the needle taper controls from about 1/4 - 3/4 throttle. The main circuit
consists of the main air jet (MAJ) and the main jet (MJ). This circuit controls the mixture
from ~1/2-2/3 throttle to wide-open throttle (WOT).
There are excellent diagrams in all three of the Inspection/Adjustment section,
the Engine/Carburetor section, and the Tuning section of the owner's manual. You
need to look in all three sections of the manual to get the full picture.
There are a few other circuits that are important but these are not usually addressed
during basic jetting for altitude and weather changes. These include the
accelerator pump (AP), the air cut valve (ACV) which is found on the WRF only, the starter jet,
and the choke circuit.
The AP provides a “squirt” of raw fuel into the carburetor venturi
when the throttle is wicked. It richens the mixture to run best at lower speeds,
yet allows a leaner top end for more over rev. This system has a jet called the
leak jet which controls how much of the AP squirt is redirected back into the
float bowl instead of into the carburetor.
The ACV prevents popping on deceleration and cannot be adjusted but it can be
bypassed.
The starter jet is used for starting (I think). I am not sure why you would change it.
The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder
walls due to condensation, the mixture is too lean for the engine to start. The
choke system will add fuel to the engine to compensate for the fuel that is
stuck to the cylinder walls. Once the engine is warmed up, condensation is not a
problem, and the choke is not needed.
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Circuit
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Components
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Throttle Position
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Idle / Pilot |
Pilot Screw, PAJ, PJ |
0 – 1/4 |
Needle |
Throttle Valve, Needle, Needle Jet |
1/4 – 3/4 |
Main |
MAJ, Main Jet |
1/2 - WOT |
The Effects of Weather / Altitude on Jetting
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At higher altitudes, the bike
runs richer (so you must jet leaner). The air is less dense at higher
altitudes (so you're essentially getting a smaller volume of air than you do
near sea level). |
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At warmer temperatures, the bike
runs richer (so you must jet leaner). During the summer, you typically want to
use a smaller MJ. During the winter, you may
have to go up on the MJ and you may want to raise the jet needle
(lower the clip). |
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At high humidity or when it's
wet, the bike also runs richer (so you must jet leaner). |
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The most sound advice is this:
If it is too rich, you foul the plug, but if it is too lean, you'll eat the
engine. Also remember that 20% of the work will correct 80% of the jetting and
get you 90% of the engine's power. That last 10% of power requires four times
as much work. |
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For a good discussion regarding climate conditions see the following TT
thread:
elevation/temperature relationship to jetting |
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TT member Mike Olichney has developed
an MS Excel spreadsheet to help adjusting for altitude and temperature. Click
here to download.
The spreadsheet is based on the following formula:
Correction Factor (CF) = 1.0778-.00111*T + (2.38554*10^-10)*A^2 - (1.0777*10^-5)*A
T is temperature in degrees F; A is altitude in feet
If you plug in A=0 and T=70 you get CF=1. This is your correction factor at
standard temperature and pressure (STP). If you put in 5000 ft and 70 F
(Denver), you get CF=.95. So your bike that was jetted for a 175 at STP now
requires a 0.95 * 175 = 166 (or the nearest size). |
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Preparing for the Jetting Experience
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Carburetor troubleshooting is simple once
the basic principles are known. The first step is to find where the engine is
running poorly. Remember the throttle position, not the RPMs, determine
which circuit is controlling the mixture. Ideally, you would select
the main jet first, then the needle and clip position, and then the pilot
circuit. If the engine is having troubles
at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely
problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle
and needle jet (most likely the jet needle) is likely the problem. If the engine
is running poorly at 3/4 to full throttle, MJ is the likely problem. While jetting the carburetor, place a piece of
tape on the throttle housing. Place another piece of tape on the throttle grip
and draw a line (while the throttle is at idle) straight across from one piece
of tape to the other. When these two lines are lined up, the engine will be
idling. Now open the throttle to full throttle and draw another line directly
across from it on the throttle housing. At this point, there should be two lines
on the throttle housing, and one on the throttle grip. Now find the half-way
point between both of the lines on the throttle housing. Make a mark and this
will show when the throttle is at half throttle. Divide the spaces up even again
until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines
will be used to quickly find the exact throttle opening while jetting. Clean the
air filter and warm the bike up.
On any circuit, if it is too lean it will pop and snap. If on the stand for
the pilot or needle circuits, you can hear these best with the seat off
listening through the air box. These are most often heard on deceleration but
are sometimes masked by a WRF air cut valve. The air cut valve should be
bypassed for proper jetting of the pilot circuit on the WRF. If you hear
popping, too lean. A bog can be rich or lean but if it is not accompanied by
popping, it is probably too rich.
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The MJ is a small brass threaded jet with a 6mm hex end and threads on the other. You
can get to it with the carburetor in place on the bike. Turn the fuel petcock
OFF. Open the float bowl drain tube and allow
the gas to drain out. Take off the 17mm hex nut at the very bottom of the
carburetor. A small amount of gas will pour out that is in the nut. If you
feel in the middle of the hole where you pulled the nut from, you will feel the
small hex nut of the MJ. A 6mm 1/4" socket can take it off. It should only be
about 1/2" long. If it is over 2" long, the needle jet came out also. No
problem, just put both back in together. Loosening the boots and rotating the
carburetor helps.
The needle is under the very top of the carburetor. On a WRF you will need to remove
the tank and seat. There are two 3mm hex bolts on the front and back of the top
cover. Clean off the top of the carburetor and the bottom of the frame above the
carburetor with mineral spirits or WD40 and a rag. Take off the two hex bolts.
Carefully remove the top of the carburetor. It has a rubber gasket attached so
try to do this without rubbing it around too much. Looking down into the
carburetor you will see the throttle slide. Twist the throttle and it will rise.
In the middle of the throttle slide is a 4mm hex screw. Remove it and it will
come out along with a spring and a collar. Don't loose the spring and collar.
Under it is the round top of the needle. Using tweezers or needle nose pliers,
gently lift it. Have someone twist the throttle to raise it up if you have
trouble getting to it. Pull it out.
The PJ is in the same hole under the 17mm nut in the bottom of the carburetor that the
MJ was in. It is in front of the main and has a flat screwdriver head. Simply
use a small short screwdriver to remove it.
The leak jet is located in the float bowl. You will need to remove the float
bowl and then use a small screwdriver to remove the jet.
The PAJ and MAJ are in the intake port of the carburetor. You will need to
remove the air boot and the intake bell to access them. You may need to take the
carburetor off for the air jets. Use a small screwdriver to remove the jets.
The fuel screw is at the bottom of the carburetor in the front (engine side) recessed
in a small hole cast into the float bowl. You can't see it and you probably
don't have a short enough screwdriver narrow enough to get in that hole. You
will probably have to make one by cutting down an existing screwdriver or use a
"carb tool" like the ones from Motion Pro.
Here are some links to some quick
adjust fuel screws and fuel screw tools you can make or purchase:
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The classic way to check the main jet is the plug test. Find a safe place at
least 1/4 mile long, preferably straight and preferably uphill. Warm up the
bike thoroughly (at least 15 minutes riding). Put in a clean plug or at least
take the current one out and notice the condition of the plug before starting.
Ride the bike at over 1/2 throttle position over the safe course. Make sure the
throttle is always at a steady position (so the accelerator pump isn't
activated) and 1/2 to full throttle position. Full throttle is recommended if a
safe enough place is found. At the end of the course, simultaneously pull in the
clutch and hit the kill switch (Do not allow the
engine to idle or coast to a stop). Remove the plug and look at the circular ceramic
below the center electrode of the plug (ignore everything else on the plug). If
it is bone white, you are too lean and require a larger MJ. If it is dark or sooty, you are too rich
and need to go down on the MJ. If
it is a light to mid tan, you are fine. If rich, replace the main jet with a
smaller one and try again. If lean, replace main jet with a larger one and try
again. While changing jets, change them one size
at a time, test run after each change, and look at the plug color after each
run. |
Jetting the Needle Circuit |
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Just change it up a clip, ride it mid-throttle on a course with changing
conditions, lower it, ride the same course, and one will feel better than the
others. Oh yeah, It will have 5 or 6 grooves around the top with a clip in
one of them. The top groove is the top clip position or clip position number 1.
The needle goes through the middle of the carburetor through the needle jet to
the main. It blocks (emulsified) fuel coming up from the main through the needle
jet. As the throttle position is increased, it blocks the main/needle jet less,
letting more gas come through. The diameter, length, and taper fine tune this
process but for now focus only on the clip position. Raising the clip (to the
top of the needle) lowers the needle to block more fuel and is leaner. Lower the
clip (toward the bottom of the needle), raises the needle out of the main/needle
jet richening the mixture. Ride it each time you change it. If it pops or is
more sluggish as you raise it, try the other way. |
Jetting the Pilot Circuit |
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When the pilot
circuit is jetted properly, starting the bike should not be a problem. You will need the choke to start cold and it won't run well off of choke for 30
seconds or so. When hot you will probably need the hot start. The bike should
idles smoothly and have no hesitation of idle and minimal backfiring on
deceleration. For more info about the pilot screw, see this
article
that recently appeared in Motocross Action Magazine.
Setting the Fuel Screw / Pilot Jet "by Ear"
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Adjust the idle with the black knob until it is too fast. Then adjust it back down
until it is around 1900-2000 RPM or if you don't have a tachometer (see below) until it sounds just a little high.
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Before you start adjusting, count the turns required to tighten it up lightly.
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Then start the bike with the slightly elevated idle and turn it out 1/4 turn, 1/2
turn, 3/4 turn and so on until you get to 2 turns. Listen for best RPM and best
response to a quick 1/4 turn tweak of the throttle at each position of the fuel
screw.
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Now turn back in 1/4 turn at a time doing the same thing. By now you should have
been able to distinguish the speed of the idle and the responsiveness to
tweaking the throttle.
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If it gets better between 3/4 and 2 turns out, set it at the best location and leave
the rest of the pilot circuit alone.
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If it is getting better turning it in or is best less than 3/4 turns out, replace the
pilot jet with a smaller one and go through this procedure again.
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If it is getting better as you turn it out or best at more than 2 turns out, replace
the pilot jet with a larger one and go through this procedure again.
Setting Fuel Screw / Pilot Jet with Tachometer
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Warm the bike up by riding about 10 minutes. Place it on a stand, have it idling. If you have a fan, direct
it into the radiators (A YZF will start to boil out if you take too long to do this,
WRFs have a nice catch tank).
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Turn the fuel screw 1 1/2 turns out. Read the RPM for
about 10 seconds (on my tachometer, cause it bounces around). If the avg RPM is not
between 1700 and 1900, adjust to about 1800 with idle screw knob on carburetor Write
down average RPM.
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Turn the fuel screw 1/2 turn out. Write down the average RPM.
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Turn the screw 3 turns out. Write down the average RPM.
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If #2 is greater than #3 or #4, you have the right pilot jet. Usually the difference will only be 50 to 100 RPM.
Go on to step #7.
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If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.
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If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.
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Adjust the fuel screw in 1/4 turn increments around 1 1/2 turns out and find the
maximum RPM fuel screw position. If the idle is now above 1900 RPM, turn it down
to be in spec.
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If you get a little deceleration backfiring on closed throttle, try adding another 1/4 turn out. Remember
you will need to redo this if the altitude or temperature changes significantly.
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Table 2: Adjustable Pilot Air Jet |
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P.J |
P.A.J |
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48 |
110-125 |
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45 |
100 |
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42 |
85 |
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40 |
70 |
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38 |
60 |
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35 |
45 |
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An adjustable pilot air jet is available form
Sudco. TT member Taffy created this chart that describes
the relationship between the PJ and the PAJ. |
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Discussion of the accelerator pump is covered in the Free mods section and in the maintenance section.<
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ThumperTalk Archives
(updated by TT member ridingagain 12/11/04) |
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There is a lot of information on the ThumperTalk message boards regarding
jetting the 250F. You can search the archives for days on end. Some of the most
informative/general threads are provided below:
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