Jetting Guide: The Yamaha / Keihin FCR Carburetor


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.


Table 1




Throttle Position

Idle / Pilot

Pilot Screw, PAJ, PJ

0 – 1/4


Throttle Valve, Needle, Needle Jet

1/4 – 3/4


MAJ, Main Jet

1/2 - WOT


Figure 1



 The Effects of Weather / Altitude on Jetting


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).

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).

At high humidity or when it's wet, the bike also runs richer (so you must jet leaner).

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.

For a good discussion regarding climate conditions see the following TT thread: elevation/temperature relationship to jetting

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).


 Preparing for the Jetting Experience


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.


 Accessing the Jets


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:

Zip-Ty Racing Fuel Mixture Screw: $21.95 + S/H

Pilot Fuel Screw Adjuster: $12.99

Motion Pro FCR Carb Tool: ~$12.80

Kouba T Handle Fuel Screw Adjuster

Sears Craftsman Stubby Bit Driver: $1.99

Custom Fuel screw


 Jetting the Main Circuit


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


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


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"

  1. 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.

  2. Before you start adjusting, count the turns required to tighten it up lightly.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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

  1. 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).

  2. 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.

  3. Turn the fuel screw 1/2 turn out. Write down the average RPM.

  4. Turn the screw 3 turns out. Write down the average RPM.

    1. 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.

    2. If #3 is greatest, you need less fuel. Install the next smallest number pilot jet. Go to step #3 and repeat.

    3. If #4 is greatest, you need more fuel. Install the next largest number pilot jet. Go to step #3 and repeat.

  5. 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.

  6. 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.


Table 2: Adjustable Pilot Air Jet

















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.


 Accelerator Pump


Discussion of the accelerator pump is covered in the Free mods section and in the maintenance section.<

Maintenance: Tuning the AP Squirt

Free Mods: AP Mods


 Air Cut Valve


Free Mods: ACV


 ThumperTalk Archives (updated by TT member ridingagain 12/11/04)


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:

James Dean's  Jetting Tips

JD Jetting High Performance Jet Kit - 2002 WR 426 to YZ jetting question. - Accelerator Pump 101 - Another Stupid Jetting Question... - Backfire - Backfire when coming off high rpm's - Bottom end bog - High Altitude jetting for YZ250F - Horsepower on order!!! - How lean is too lean - Is crackling on deceleration normal - JD Jetting Kit ride report - Jetting 101 - Jetting Qs - Leak Jet Mod - leak jet mod to eliminate bog - Plug Chop Test (Myth) Debate... - Plug Color Questions - How To - Problem with a-f mix at idle - RPM's Stay High - Spark plug reading - Want More Mid-Range Power - Weekly Jetting Question - WR to YZ carb mod - Rich or to tell - rich or lean - Jetting needles


 Other Links


James Dean's  Jetting Guide (spreadsheet to help predict changes in jetting)

Also see the Carb Theory Links on the Links page

Motocross Action Magazine: The Secret World of the Fuel Screw

Motocross Action Magazine: Thumper's Fuel Screw

Motocross Action Magazine: The Art of Jetting



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Last Updated 101/04/2005