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Discussion Starter · #1 ·
I found this on PStuning.com and it made sense to me so i'm reposting it here.. With all the questions I have had and have seen about jetting and carb tuning in the last couple weeks.


Jetting from the bottom to the top!

To properly jet your carburetor the carb must be clean and with sound gaskets, seals and o-rings. Vent tubes must be clean and free flowing. Disassemble and clean the carb and all the passageways with carb cleaner and compressed air.

The engine must be in sound mechanical and electrical condition with no air leaks. So… do a compression check along with a pressure check to make sure you have a good foundation. A new spark plug with the correct gap and properly set ignition timing is also helpful. You can tune till you're blue in the face, if you don't have a clean carb, with a mechanically sound engine as a baseline.

Both Mikuni and Keihin’s recommended jetting procedure is to start from the bottom and proceed to the top. A properly set float level is critical to all circuits in the carburetor and it must be set first. A simple and fairly accurate method to set the float level… is with the float bowl removed, tilt the carb body to about a 45° angle and watch the float needle. Most have a spring-loaded needle… what you are looking for is the point where the float tab just meets the needle valve, without compressing it. Continue to tilt the carb to the point where the spring just barely touches, and then look at the float casting marks found on most floats. With a properly set float the marks will typically be parallel to the body of the carburetor. This point is also the point to take your float level measurement and with the float in this position the distance should equal the required measurement, per the manufacture’s specifications. If not… carefully bend the tab that the float needle rides on to get the correct level.

Please remember that we tune each of the carburetor circuits based on throttle position and not RPM. We also go from the bottom to the top, because to varying degrees there is a culmination/overlap of mixtures from one circuit/position to the next one. If these overlaps were not present and correctly tuned for… you would experience flat spots or bogs and/or surges as you open the throttle.
Idle to about ¼ throttle opening is primarily controlled by the pilot circuit and this fuel/airflow continues all the way to WOT… and thus affects the whole jetting spectrum. This is one of the major reasons that you jet from the bottom to the top.

1/8 to about 3/8 is controlled by the slide valve cut away.
¼ to about ¾ is controlled by the needle jet and jet needle.
5/8 to WOT is controlled by the main jet.
Note how each circuit overlaps!

An important point to note is that if a particular circuit is either too rich or too lean… the amount of overlap can shift up/down, shrink/expand the range that it should normally control. Due to an engines design characteristics, rider driving technique and a multitude of other variables… a particular circuit may need to be richer/leaner than it would be in a different situation. Then an adjustment would have to be made on either side to make the transition smooth.

The transition from one area to the next doesn't have a Red or Green light…it has a blinking yellow, and just like in driving, you need to proceed with caution. If you start from the bottom and go up… you’re tuning for these variables as you open the throttle… allowing the next step to be more defined. Remember… that’s what we’re looking for… smooth transition from bottom to top, with the correct mixture ratio. Then as you roll off the throttle you'll have a smooth transition back down the range also. This is just as important! A lot of two stroke engines start to seize shortly after you come out of the throttle, not when you are WOT! Why… because when you shut off the gas, you also shut off the oil, and after a long WOT run you have built up a lot of heat, and you need the gas/oil mixture to protect your piston/cylinder walls.
 

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Discussion Starter · #2 ·
Now that we have the float level set and have a basic understanding of how the circuits affect each other… let’s just do it!

The pilot circuit is controlled by the pilot jet and air/fuel adjusting screw. If the adjustment screw is on the engine side of the slide it controls the fuel, and if it’s on the air cleaner side it controls the air. Most MX style carburetors have the adjusting screw on the air cleaner side, so it controls the airflow. A simple way to look at this circuit is the pilot jet controls the fuel and the air adjusting screw allows you to add the appropriate amount of air to get the proper air/fuel mixture.

To tune the pilot circuit… lightly seat the adjusting screw by turning it clockwise till it just touches the seat. Don’t tighten it too snug or you might damage the seat or the taper on the end of the adjusting screw. Now back it out to a base line setting of 1-½ turns. Warm up the engine to a normal operating temperature. Adjust the idle speed adjusting screw so that you are about 500 RPM higher than your normal idle speed. Make a mental note of the air adjusting screws location, and slowly turn the adjustment screw in, and then turn it out in 1/8-turn increments, until you have found the highest idle RPM. Go slowly and let the RPM stabilize. Go back and forth a couple of times so that you get it spot on. If the circuit is adjusted properly you will have a smooth idle and the transition from idle to about ¼ throttle will be also be smooth, with no hesitations or flat spots.

Note how many turns it took on the adjusting screw to get your best idle speed. If you had to turn the screw more than one turn out from the baseline of 1½ turns, then the pilot jet is too large and you need to go down one size on the pilot jet. Now go back through the adjustment procedure again to get the adjustment screw back to the base line setting of around 1½ turns out, when you have the best idle. Most adjustment screws have a taper that works best between 1/2 to 2 1/2 turns. Using this method allows you to make minor air adjustments to compensate for small changes in weather conditions at the track and still be in the working range of the air screw taper and spring.

The next rung up the ladder is the throttle side cut away, which controls the amount of air allowed to flow through the carb at throttle opening in the 1/8th to ½ throttle range with the most affect in the 1/8th to ¼ range. Typically… slide adjustment are not required, but… you might run in to a situation where you need to change to a leaner or richer cut away. The higher the number of the slide the more air it will flow within it’s operational range. Slides are numbered in millimeters of cut away at the closed throttle position. A number 6 slide has a 6 MM cut away and a richer number 5 has a 5 MM cut away. While you can typically adjust the pilot and slide with the engine running and the vehicle still… a good on the track test is also recommended. Accelerate out of a slow, first gear corner to give it a good test. The slide works in close conjunction with the pilot circuit to allow you to transition to the next tunable area, which is the jet needle and needle jet.

We want a smooth transition from the throttle slide to the main jet. This transition is controlled by the needle jet and jet needle. When completed, we want the jet needle’s (more commonly called the needle) clip to be in the middle position, if possible, for our base line setup.

The needle is tapered, and has groves cut in one end, to hold a clip. This clip allows you to change the length of the needle. There are also small, thin spacers available, that will let you set the needle length even more precisely, by placing them under the clip.

The needles length, diameter and taper, in conjunction with the needle jet control the rate of fuel flow in each position of the needle from a little less than ½ throttle to a little more than ¾ throttle.

Needle jets come in a couple of different styles and of course numerous jet sizes. The provide additional avenue to fine tune your mid range. Some tuners find it more convenient to replace the needle jet with one size richer instead of raising the needle. This way you don’t have to take the top off and mess the springs, etc. This way your needle will stay in middle clip setting all the time. Different strokes for different folks!
 

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Discussion Starter · #3 ·
Keihin and Mikuni use a different system for numbering their needles and you need to become familiar with whichever carb you are running. Give Sudco’s web site a hit and look through their needle info to find out what might fit your particular situation. You can get the mid range perfect if you take your time and have a selection of needles and needle jets, which by the way, is sometimes called an emulsion tube. Sudco also sell a tuning manual for Mikuni that it great… for about $12.

Ok... let's spend some time talking about the big three in needles!

The needle diameter controls the lower end of the mid range jetting... just above the pilot jet/slide area, and you feel it in the ¼ to ½ throttle range. The setting of the needle diameter is critical to the engines lower mid range power and drivability. When the needle diameter is too large or lean… the engine could have a “lean bog” just as it starts to pull well. When the needle diameter is too rich, you'll normally suffer a “rich bog” in the ¼ to 1/2 throttle position.
Ok…what’s the difference between a rich bog and a lean bog… A lean bog is a sudden loss of power with the characteristic boooooooooooowwwwwwwwww exhaust and intake sound, and if it’s way too lean… you will not be able to pull through it without letting off of the throttle a little bit. A rich bog has a much more mellow sound along with a blubber, stutter feeling and you will typically pull on through the range… in time. It should be noted that either one of these situations can be caused by your fuel delivery system or by clogged vent tubes.

Moving on now to the overall length of the needle. On most needles there are 5 clip positions. The top clip setting is called the #1 position and it’s the leanest. The length setting covers a wide range of your mid throttle jetting, with an emphasis at ½ throttle. The setting of the needle length is critical to the engines mid range power and drivability… very similar to the diameter…just a little more up in the range of the throttle position. When the needle is too long… the engine could possibly have a “lean bog” in the mid range, and conversely, when the needle diameter is too rich the machine will normally suffer a “rich bog” in the mid throttle position. To richen the circuit you have to raise the needle by lowering the clip.

The needle taper is the angle of the needle at its lower half. This part of the needle fits into the main jet. The taper works the transition between the midrange and full throttle/main jet (approximately ¾ throttle position). The taper also affects to a small degree, 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.

Selecting the proper needle takes some time but the rewards are a stronger, smooth power flow and great drivability.

The best way to select the needle is to start with the one that came with your carb. Then determine what area of the needle needs to be changed, if any. Place the needle clip in the middle position and make sure that the main jet is a size or two larger than you think will be the final size. Then make some runs at various mid throttle settings to get a feel for what needs to be changed. Some carburetors don’t have a replaceable needle jet. On those that do… it is a very effective tuning device, as you can make small adjustments in the needle jet to get a centered needle clip.

The main jet is selected in a process similar to the needle. Select a size that you feel is a little large and make some WFO high speed runs in 5th or 6th gear and then do a good plug chop to see which way, if any you need to adjust. Plug reading is a Black Art… that requires some experience in determining the condition of not only jetting, but also ignition timing and the overall condition of your engine. This is covered in another article.

Run a few laps with varying throttle position and verify that the carb flows smoothly from the bottom to the top. If any area needs fine-tuning… make the adjustments. This is the seat of the pants method.
 

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Now that we have the float level set and have a basic understanding of how the circuits affect each other… let’s just do it!

The pilot circuit is controlled by the pilot jet and air/fuel adjusting screw. If the adjustment screw is on the engine side of the slide it controls the fuel, and if it’s on the air cleaner side it controls the air. Most MX style carburetors have the adjusting screw on the air cleaner side, so it controls the airflow. A simple way to look at this circuit is the pilot jet controls the fuel and the air adjusting screw allows you to add the appropriate amount of air to get the proper air/fuel mixture.

To tune the pilot circuit… lightly seat the adjusting screw by turning it clockwise till it just touches the seat. Don’t tighten it too snug or you might damage the seat or the taper on the end of the adjusting screw. Now back it out to a base line setting of 1-½ turns. Warm up the engine to a normal operating temperature. Adjust the idle speed adjusting screw so that you are about 500 RPM higher than your normal idle speed. Make a mental note of the air adjusting screws location, and slowly turn the adjustment screw in, and then turn it out in 1/8-turn increments, until you have found the highest idle RPM. Go slowly and let the RPM stabilize. Go back and forth a couple of times so that you get it spot on. If the circuit is adjusted properly you will have a smooth idle and the transition from idle to about ¼ throttle will be also be smooth, with no hesitations or flat spots.

Note how many turns it took on the adjusting screw to get your best idle speed. If you had to turn the screw more than one turn out from the baseline of 1½ turns, then the pilot jet is too large and you need to go down one size on the pilot jet. Now go back through the adjustment procedure again to get the adjustment screw back to the base line setting of around 1½ turns out, when you have the best idle. Most adjustment screws have a taper that works best between 1/2 to 2 1/2 turns. Using this method allows you to make minor air adjustments to compensate for small changes in weather conditions at the track and still be in the working range of the air screw taper and spring.

The next rung up the ladder is the throttle side cut away, which controls the amount of air allowed to flow through the carb at throttle opening in the 1/8th to ½ throttle range with the most affect in the 1/8th to ¼ range. Typically… slide adjustment are not required, but… you might run in to a situation where you need to change to a leaner or richer cut away. The higher the number of the slide the more air it will flow within it’s operational range. Slides are numbered in millimeters of cut away at the closed throttle position. A number 6 slide has a 6 MM cut away and a richer number 5 has a 5 MM cut away. While you can typically adjust the pilot and slide with the engine running and the vehicle still… a good on the track test is also recommended. Accelerate out of a slow, first gear corner to give it a good test. The slide works in close conjunction with the pilot circuit to allow you to transition to the next tunable area, which is the jet needle and needle jet.

We want a smooth transition from the throttle slide to the main jet. This transition is controlled by the needle jet and jet needle. When completed, we want the jet needle’s (more commonly called the needle) clip to be in the middle position, if possible, for our base line setup.

The needle is tapered, and has groves cut in one end, to hold a clip. This clip allows you to change the length of the needle. There are also small, thin spacers available, that will let you set the needle length even more precisely, by placing them under the clip.

The needles length, diameter and taper, in conjunction with the needle jet control the rate of fuel flow in each position of the needle from a little less than ½ throttle to a little more than ¾ throttle.

Needle jets come in a couple of different styles and of course numerous jet sizes. The provide additional avenue to fine tune your mid range. Some tuners find it more convenient to replace the needle jet with one size richer instead of raising the needle. This way you don’t have to take the top off and mess the springs, etc. This way your needle will stay in middle clip setting all the time. Different strokes for different folks!
Is the pilot jet air fuel adjustment in the carb or the outside thanks
 
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