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Discussion Starter #1
This was written for Keihin carbs, but much of it can be used for the Z400's Mikuni. (Gives some good tuning tips)


AIR SCREW: The air screw is a small (5mm in diameter) slotted brass adjustment screw located on the inlet side (air cleaner) of the carburetor. The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The airscrew works with the pilot/slow speed system of the carburetor, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feed back on the necessary setting required for the pilot jet. The airscrew is adjusted in a rather straightforward manor.
The ideal procedure for setting the screw in the correct position is to warm up your ATV engine to the proper operating temperature. Then turn the idle up so it is idling about 500 RPM's higher than normal. Next turn the airscrew all the way in until it bottoms out, once bottomed out slowly back the screw out a ¼ turn at a time (give the engine 10-15 seconds between each ¼ turn of the screw, to allow the engine to catch up with the adjustments). Continue backing the airscrew out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from 0-1 turns out this means the pilot setting is on the Lean side and a larger pilot jet should be installed. If the engine idles at its highest RPM at over 2 turns out, this means the pilot setting is on the Rich side and a smaller pilot jet should be installed.
If you get no RPM fluctuation when adjusting the air screw there is a very realistic chance that there is something clogging the pilot/slow speed system. Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled.
If the airscrew adjustment process is unsuccessful and leaves you confused. Set the screw at 1 ½ turns out and consult a professional for further assistance.

PILOT JET: The pilot jet is a medium size (¾-1") brass jet located inside the float bowl next to the needle jet/main jet location. The pilot jet meters the fuel required for engine starting, idling and the initial throttle opening 0-1/8.
A lean pilot jet setting will cause your engine to surge at very low RPM's, bog or cut-out when the throttle is opened quickly and have trouble idling down.
A rich pilot setting will result in hard starting, plug fouling at low RPM's, sputtering as the throttle is cracked opened.
The pilot jet is not difficult to set. With proper air screw adjustment and a close initial setting from your engine tuner, fine-tuning should be painless. Once set the pilot jet is not terribly sensitive. You should only be required to adjust the setting when confronted with large weather changes or altitude swings of over 2000 ft.
If adjusting the pilot jet gives inconsistent feed back, or does unexplainable things. Check and clean out the pilot/slow speed system thoroughly with contact cleaner and blow out with compressed air.
Pilot jet sizes are numbered in the following pattern; #42, #45, #48, #50, #52, #55, #58, #60 etc. repeating the pattern. Sizes available on most models are from #35 to #80.

SLIDE: The slide not only monitors how much airflow goes into your engine (its main job). But it has various angles cut on the bottom of the slide to monitor airflow at low RPM's. This is referred to as slide cut away. The slide cut away is measured in 4.0, 5.0, 6.0 etc. (see attached chart). The higher the number, the larger the cut away the leaner the slide setting is.
The slide cut away generally effects the jetting in the ¼ throttle range at almost the same throttle position as the needle diameter effects. The slide cut away is usually predetermined by the engine manufacture or engine tuner. As a general rule do not change the slide cut away unless instructed to do so by a skilled engine tuner.
For ¼ throttle jetting adjustments it is easier to adjust the needle diameter.

NEEDLE: The jet needle is the most important component in determining your carburetors jetting. The needle is broken into 3 main functions; Diameter, Length, Taper. These needle functions have a large effect on the carburetors jetting from ¼ to ¾ throttle. In the following paragraphs we will explain the needles functions and how to adjust them.
DIAMETER: The needle diameter controls the jetting just above the pilot jet, right as the engine begins to pull. On most engine combinations the needle diameter is felt in the ¼ throttle range. The setting of the needle diameter is crucial to both the engines low RPM power and reliability.
The jetting at ¼ throttle is adjusted by changing the diameter of the needle. On gold colored needles identified with the 3 stamped in letter I.D. system the last letter refers to the needle diameter size. By referencing the enclosed jetting chart you can verify your needles size, and be able to determine what needle size may be required for your specific situation. In many instances you can leave the taper and length settings the same (if they are correct) and adjust only the diameter. EXAMPLE: If you have a needle marked DGJ and change it for a needle marked DGK, you have effectively Leaned the jetting at the ¼ throttle position. Reference the enclosed jetting chart to clearly understand this adjustment.
When the needle diameter is Lean the machine will have a loss of low-end power. The engine will feel very zingy (like a 125cc engine). When an engine is in this condition and then put under a heavy load the engine becomes very susceptible to seizing.
When the needle diameter is Rich the machine will sputter at ¼ throttle and be hesitant to take the throttle. In extreme cases the engine can feel like the choke is on or the plug is fouling.
When the needle diameter jetting is set correctly the engine will accelerate evenly thru the first part of the power band. The proper diameter setting will provide maximum low RPM power and very ride able throttle response.
It is important to remember that even though the needle diameter is mainly responsible for the jetting at ¼ throttle there is some bleed effect. With experience this can easily be deciphered. An excellent way to pin point the feel of the needle diameter is test needles in your machine that have both the same taper and length but richer and leaner diameter settings. Try a needle of each setting in your machine for 10-15 minutes of riding and you will begin to understand specifically what throttle position your dealing with.
LENGTH: The needle length is determined by the clip position (grooves at top of needle) setting on the upper portion of the needle. On most needles there are 5 clip positions. The top clip position is referred to as #1 and is the Leanest setting. The clips are referred to in numerical order with the bottom position being #5, the Richest (refer to attached jetting chart illustration). The clip/length setting covers the largest percentage of jetting in your carburetor. With an emphasis at ½ throttle, the clip (length) setting will bleed both up and down to some degree to cover a wide portion of the midrange jetting.
When the clip/length setting is Lean the machine will be very zingy sounding and feel kind of similar to an 80cc or 125cc machine. Lean in the midrange will also rob power and cause the machine to run hot and seize easily
When the clip/length setting is Rich the machine will have a lazy feeling in the midrange. Exhaust note will be a little flat sounding. In extreme cases of richness the engine will even sputter or kind of crap out in the midrange.
The safest way to set the clip position is to richen up the clip position setting until the machine loses a little power (feels lazy/unresponsive) then lean it back one position. Ideally you like to run the needle setting in either the 3rd or 4th clip position, if possible. The needle clip jetting is especially critical to your machines reliability because on average more time is spent in the midrange than any other part of the throttle. Most machines pull very hard in the midrange, putting quite a load on the engine. This makes a lean condition very detrimental to your reliability.
TAPER: The needle taper is the angle of the needle at its lower half. The taper works the transition between the midrange and full throttle/main jet (¾ throttle position). The taper is the least sensitive function of the needle. Changes in the taper have very mild subtle changes in the jetting. The taper also affects the main jet size your carburetor requires. A leaner needle taper will use a richer main jet than a comparable engine/carburetor combo with a richer needle taper.
As a general rule, your engine tuner or engine manufacture should preset the taper. Once set correctly by a professional the taper setting should not need to be changed except for cases of extreme temperature reduction.

MAIN JET: The main jet affects the jetting in the upper quarter of the throttle position. Coming into play at ¾ throttle on through to full open throttle. Even though most people relate the main jet to their carburetor in general. The main jet is only responsible for the last ¼ of the jetting. The main jet does not effect the jetting for starting and idling. It plays no part on low RPM or mid RPM jetting either. The main jet is very important to your machines overall tuning, but should never be over emphasized at the expense of needle tuning or other facets of your carburetion tuning.
When the main jet is Lean the engine will experience detonation or "pinging". Exhaust note will be of a higher, tinier type note. Engine will over heat easy and can be down on horsepower. A moderately lean main jet can cause engine seizures. A severely lean main jet can cause the engine to burn a piston (whole in top).
When the main jet is Rich the engine will be a bit flat or lazy at ¾ to full throttle, giving off a flat, dead sounding exhaust note. When the main jet is severely rich the engine will sputter in the high RPM's and have a lot of trouble making power up top.
The safest way to get the main jet setting as near correct as possible is to richen the main jet setting up until the engine begins to lose power and not rev to as high of RPM as before. On a single cylinder machine this will signal that the jetting is beginning to get rich. Depending on your riding application you can lean it down a bit from there or leave it for conditions requiring extra fuel (desert racing, long high speed runs, etc.)
As a general rule, richen the jetting up as long as the engine likes it and continues to run just as well or better than the smaller size main jet previously installed. When the engine no longer continues to improve its performance you will know you have gone to far.
Main jets are offered from #90 to #230. Starting at #90, sizing cycles like the following #90, #92, #95, #98, #100, #102, #105, #108, #110, #112, etc…. up thru #200, #205, #210, #215, #220, #230
 

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Just found this thread in search...probably should be a sticky'd thread with how many questions about jetting get posted on here. The part explaining the needles function I found to be especially helpful.
 
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it should be noted that although its a great writeup, its for 2 stroke carb so some things are a little different for us, mainly the "air screw" is what we know to be the pilot screw, and it controls fuel not air.
 

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4Strokes.com Technical: Four-Stroke Carb Jetting Tricks
Your engine is basically an air pump, and your carb meters how much air and fuel are sucked into that pump. Even though they may differ wildly in size, shape and design, all four-stroke carburetors have the same basic parts or circuits. Your slide cutaway (or throttle valve) needle and needle jet will all affect your bike's acceleration from one-quarter to three-quarters throttle, and this is the most important area for off-road riders, since we spend the most time at these throttle settings. Due to the hassle of making changes to these circuits, these are the most neglected areas of tuning. Too rich jetting (too much cutaway, needle positions too high, too large a needle jet) can make your bike lunge and hard to control. If it's too lean in this area, the bike will feel really flat and down on power, but will respond quickly to changes in throttle position. It may detonate (ping) under a load too. Pinging can also be caused by too little octane or winterized fuel (oxygenated, blended with additives), so keep in mind any fuel changes if your bike suddenly starts detonating in otherwise "normal" conditions.


Your main jet is probably the most talked-about circuit, and it's as critical to get it right on a four-stroke as with a two-stroke. The main kicks in at half throttle and takes over metering duties as you hit full throttle. If your main is too rich, the bike will sputter and surge as it tries to burn all of that fuel. Too lean, and the bike will run flat or have a flat spot in the powerband. A severely lean main will cause your bike to seize just like a two-stroke. It's better to be slightly rich on the main than slightly lean, because it will run cooler.


Yamaha's new 400s have an accelerator-pump circuit. This system squirts a stream of raw fuel into the carb venturi every time you wick the throttle. Think of it as the four-stroke's PowerJet carb - it richens the mixture to run best at lower engine speeds, yet allows a leaner top for more over-revs. If you radically modify your engine (flowed head, hot cam, etc.), you may have to richen this circuit slightly, but it's otherwise not something you mess with for mere weather or altitude changes.


Your pilot jet (or slow jet) controls the idle circuit, or from zero to one-quarter throttle opening. The pilot jet and airscrew control the amount of fuel and air going into the engine at slow engine speeds. It's very important to tune these circuits because they control throttle response and starting. The pilot circuit has a major affect on how well your four-stroke starts -or refuses to start - after a fall. At every event we attend, there is always some four-stroke rider who comes into the pits with his bike revving wildly. Invariably, this rider will say that his bike is hard to restart after a stall, so he turns up the idle adjuster so it won't die.


That's like jumping from the frying pan in to the fire. Thumpers are only hard to start when they are jetted poorly or when the wrong technique is used. The rider who turns up his idle is only perpetuating the myth about thumpers being hard to start. Most manuals (and this magazine) tell you that you should not touch the throttle when you kick a thumper. Well, turning the idle up is mechanically opening the throttle, right? You will make, your bike even harder to start. You have to fix the problem, not the symptoms of the problem!


Carburetor Jetting Tricks
Technical Article Sponsor

General Carburetor Jetting Tricks
Your bike’s owner's manual is a great source for recommended jetting and tuning tips. If you bought your thumper used and don't gave a manual, get one. Set the idle speed as per your manual. If it won't start easily using the manual's technique, your pilot jet is the likely culprit.


Whether your bike is air or water cooled, you should start it and get it up to race temperature before tuning the pilot circuit. A hotter engine will run leaner than an old one, so failure to properly warm the bike will result in a too-rich setting. With the bike up to temp, adjust the airscrew so that the bike runs and responds best to slight throttle movements. Now, kill the motor and see how many turns out you have on the airscrew. Less than one, and your pilot is too lean. More than two, and it's too rich. Install the next-size pilot and repeat the test.


Most off-road bikes are jetted lean to meet emissions standards, so you will likely want to richen these circuits, especially if you have gone to an after-market pipe, air filter or even removed OEM baffles (pipe and/or airbox). If you remove the muffler diffuser, you should toss the airbox stuffer too, or the airbox won't be able to draw enough air to feed the engine. Most aftermarket companies will give you recommended jetting, so use this as a baseline.


Under most conditions, about the only time you will need to go leaner on an EPA-legal four-stroke is because of altitude. Air is thinner at higher altitudes, so it contains less oxygen, and your jetting will be too rich. You will want to go down a size on the pilot, one or two on the main and lower the needle a position (raise the clip).


Cold air is denser than warm air, so it holds more oxygen. On cold mornings, your jetting will be slightly rich, but thumpers are less susceptible to changes than two-strokes. Where you might change the pilot on a two-stroke when it's really cold, an airscrew adjustment will suffice on a thumper.


The same is true for barometric pressure. As the barometer rises, the pressure compresses the air, and your jetting will be slightly lean. A falling barometer causes a rich condition, but thumpers don't care about the weather as much as two-strokes.


Four-Stroke Carburetion Troubleshooting
Overall, the Yamaha YZ400F is jetted almost perfectly from the factory; however, it is very picky about its air filter. Do not over-oil the filter, and do not expect it to start immediately after oiling the filter. Let it sit overnight (not in the cold) to allow the carriers to evaporate. Better yet, keep spare filters in a plastic bag so that you never put a freshly oiled filter in the bike on race day. Modifications throw stock jetting out the window, so this troubleshooting guide will apply to the 400F as much as any other four-stroke.


Bike Won't Start After a Crash
  • <LI class=txt>Pilot too lean <LI class=txt>Idle set too high <LI class=txt>Improper starting procedure
  • Bike wants hot-start button (KTMs and 400Fs)
Bike Runs-On or Won't Idle Down When Throttle is Chopped
  • <LI class=txt>Idle set too high <LI class=txt>Air leak in intake or engine
  • Pilot too rich (when bike is hot)
Bike Wont Start When Cold Temp Outside
  • <LI class=txt>Pilot jet too lean <LI class=txt>Air filter over-oiled
  • Motor oil too thick for temperature
Carburetor Jetting Tricks
Technical Article Sponsor

Bike Sputters / Wont Clean Out at High RPM
  • <LI class=txt>Main jet too rich <LI class=txt>Air filter over-oiled
  • Spark plug has debris on electrode
Bike Coughs & Stalls in Slow Turns
  • <LI class=txt>Pilot jet too lean <LI class=txt>Idle set too low <LI class=txt>Valves set too tight
  • Decompressor is set too tight, so turning the bars engages release slightly
Bike Hesitates or Bogs Over Deep Whoops or G-Outs
  • <LI class=txt>Float level too low <LI class=txt>Carb vent tubes blocked <LI class=txt>Main jet splash shield not installed
  • Float level too high, gas is trapped in vent tunes (install T-vents)
Bike Starts But Wont Take Throttle Without Sputtering
  • <LI class=txt>Pilot jet too rich <LI class=txt>Water in fuel
  • Debris in main jet
Bike Suddenly Starts Sputtering / Gas Flows from Vent Tubes
  • <LI class=txt>Stuck float check valve
  • Debris in gas or carb
Bike Runs Hot / Feels Slow & Flat on Straights
  • <LI class=txt>Main jet too lean
  • Fuel octane too low, causing detonation
Bike Coughs & Stalls When Throttle is Whacked Open
  • <LI class=txt>Needle too lean <LI class=txt>Slide cutaway too lean
  • Pumper circuit blocked or too lean
PDF version of this article: Four-Stroke Jetting Tricks
 

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This was a cut and paste but the pdf version is linked at the bottom.

4Strokes.com Technical: Four-Stroke Carb Jetting Tricks
Your engine is basically an air pump, and your carb meters how much air and fuel are sucked into that pump. Even though they may differ wildly in size, shape and design, all four-stroke carburetors have the same basic parts or circuits. Your slide cutaway (or throttle valve) needle and needle jet will all affect your bike's acceleration from one-quarter to three-quarters throttle, and this is the most important area for off-road riders, since we spend the most time at these throttle settings. Due to the hassle of making changes to these circuits, these are the most neglected areas of tuning. Too rich jetting (too much cutaway, needle positions too high, too large a needle jet) can make your bike lunge and hard to control. If it's too lean in this area, the bike will feel really flat and down on power, but will respond quickly to changes in throttle position. It may detonate (ping) under a load too. Pinging can also be caused by too little octane or winterized fuel (oxygenated, blended with additives), so keep in mind any fuel changes if your bike suddenly starts detonating in otherwise "normal" conditions.


Your main jet is probably the most talked-about circuit, and it's as critical to get it right on a four-stroke as with a two-stroke. The main kicks in at half throttle and takes over metering duties as you hit full throttle. If your main is too rich, the bike will sputter and surge as it tries to burn all of that fuel. Too lean, and the bike will run flat or have a flat spot in the powerband. A severely lean main will cause your bike to seize just like a two-stroke. It's better to be slightly rich on the main than slightly lean, because it will run cooler.


Yamaha's new 400s have an accelerator-pump circuit. This system squirts a stream of raw fuel into the carb venturi every time you wick the throttle. Think of it as the four-stroke's PowerJet carb - it richens the mixture to run best at lower engine speeds, yet allows a leaner top for more over-revs. If you radically modify your engine (flowed head, hot cam, etc.), you may have to richen this circuit slightly, but it's otherwise not something you mess with for mere weather or altitude changes.


Your pilot jet (or slow jet) controls the idle circuit, or from zero to one-quarter throttle opening. The pilot jet and airscrew control the amount of fuel and air going into the engine at slow engine speeds. It's very important to tune these circuits because they control throttle response and starting. The pilot circuit has a major affect on how well your four-stroke starts -or refuses to start - after a fall. At every event we attend, there is always some four-stroke rider who comes into the pits with his bike revving wildly. Invariably, this rider will say that his bike is hard to restart after a stall, so he turns up the idle adjuster so it won't die.


That's like jumping from the frying pan in to the fire. Thumpers are only hard to start when they are jetted poorly or when the wrong technique is used. The rider who turns up his idle is only perpetuating the myth about thumpers being hard to start. Most manuals (and this magazine) tell you that you should not touch the throttle when you kick a thumper. Well, turning the idle up is mechanically opening the throttle, right? You will make, your bike even harder to start. You have to fix the problem, not the symptoms of the problem!


Carburetor Jetting Tricks
Technical Article Sponsor

General Carburetor Jetting Tricks
Your bike’s owner's manual is a great source for recommended jetting and tuning tips. If you bought your thumper used and don't gave a manual, get one. Set the idle speed as per your manual. If it won't start easily using the manual's technique, your pilot jet is the likely culprit.


Whether your bike is air or water cooled, you should start it and get it up to race temperature before tuning the pilot circuit. A hotter engine will run leaner than an old one, so failure to properly warm the bike will result in a too-rich setting. With the bike up to temp, adjust the airscrew so that the bike runs and responds best to slight throttle movements. Now, kill the motor and see how many turns out you have on the airscrew. Less than one, and your pilot is too lean. More than two, and it's too rich. Install the next-size pilot and repeat the test.


Most off-road bikes are jetted lean to meet emissions standards, so you will likely want to richen these circuits, especially if you have gone to an after-market pipe, air filter or even removed OEM baffles (pipe and/or airbox). If you remove the muffler diffuser, you should toss the airbox stuffer too, or the airbox won't be able to draw enough air to feed the engine. Most aftermarket companies will give you recommended jetting, so use this as a baseline.


Under most conditions, about the only time you will need to go leaner on an EPA-legal four-stroke is because of altitude. Air is thinner at higher altitudes, so it contains less oxygen, and your jetting will be too rich. You will want to go down a size on the pilot, one or two on the main and lower the needle a position (raise the clip).


Cold air is denser than warm air, so it holds more oxygen. On cold mornings, your jetting will be slightly rich, but thumpers are less susceptible to changes than two-strokes. Where you might change the pilot on a two-stroke when it's really cold, an airscrew adjustment will suffice on a thumper.


The same is true for barometric pressure. As the barometer rises, the pressure compresses the air, and your jetting will be slightly lean. A falling barometer causes a rich condition, but thumpers don't care about the weather as much as two-strokes.


Four-Stroke Carburetion Troubleshooting
Overall, the Yamaha YZ400F is jetted almost perfectly from the factory; however, it is very picky about its air filter. Do not over-oil the filter, and do not expect it to start immediately after oiling the filter. Let it sit overnight (not in the cold) to allow the carriers to evaporate. Better yet, keep spare filters in a plastic bag so that you never put a freshly oiled filter in the bike on race day. Modifications throw stock jetting out the window, so this troubleshooting guide will apply to the 400F as much as any other four-stroke.



Bike Won't Start After a Crash
  • <LI class=txt>Pilot too lean <LI class=txt>Idle set too high <LI class=txt>Improper starting procedure
  • Bike wants hot-start button (KTMs and 400Fs)
Bike Runs-On or Won't Idle Down When Throttle is Chopped
  • <LI class=txt>Idle set too high <LI class=txt>Air leak in intake or engine
  • Pilot too rich (when bike is hot)
Bike Wont Start When Cold Temp Outside
  • <LI class=txt>Pilot jet too lean <LI class=txt>Air filter over-oiled
  • Motor oil too thick for temperature
Carburetor Jetting Tricks
Technical Article Sponsor


Bike Sputters / Wont Clean Out at High RPM
  • <LI class=txt>Main jet too rich <LI class=txt>Air filter over-oiled
  • Spark plug has debris on electrode
Bike Coughs & Stalls in Slow Turns
  • <LI class=txt>Pilot jet too lean <LI class=txt>Idle set too low <LI class=txt>Valves set too tight
  • Decompressor is set too tight, so turning the bars engages release slightly
Bike Hesitates or Bogs Over Deep Whoops or G-Outs
  • <LI class=txt>Float level too low <LI class=txt>Carb vent tubes blocked <LI class=txt>Main jet splash shield not installed
  • Float level too high, gas is trapped in vent tunes (install T-vents)
Bike Starts But Wont Take Throttle Without Sputtering
  • <LI class=txt>Pilot jet too rich <LI class=txt>Water in fuel
  • Debris in main jet
Bike Suddenly Starts Sputtering / Gas Flows from Vent Tubes
  • <LI class=txt>Stuck float check valve
  • Debris in gas or carb
Bike Runs Hot / Feels Slow & Flat on Straights
  • <LI class=txt>Main jet too lean
  • Fuel octane too low, causing detonation
Bike Coughs & Stalls When Throttle is Whacked Open
  • <LI class=txt>Needle too lean <LI class=txt>Slide cutaway too lean
  • Pumper circuit blocked or too lean
PDF version of this article: Four-Stroke Jetting Tricks
 
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