Post by mellow on Sept 6, 2011 18:07:08 GMT 1
I know there are other posts about jetting but i robbed this post from elsewhere on the net. I thought it was very good and quite easy to understand with symptons of what causes what 
Name It Affects
1) Pilot jet 1/16 to 1/4 throttle
2) Air screw Idle to just off of idle (not the idle screw)
3) Jet needle 0-1/2 throttle, work with the needle shape first, then the clip position
4) Needle jet 1/8-3/4 throttle, also called nozzle
5) Slide cutaway 1/4-3/4 throttle, the angle of the cut affects air intake (high-dollar item)
6) Main jet 1/2-full throttle
1) Pilot jet - Affects mixture from idle to 1/3 throttle opening. The pilot jet meters fuel to the "bottom end circuits"
2) Air screw - Meters air to pilot jet. It is usually located near the back or air box side of the carb. Turning in clockwise will richen the pilot mixture. Turning out, counter clockwise, will lean out the pilot mixture. Average setting for most two strokes is 1 to 2 1/2 turns out. Refer to owner's manual. If the airscrew is set to less than 1 turn, it needs a bigger pilot.
3) Needle - Affects the mixture from 1/4 throttle to full throttle. The needle is in the leanest position when the clip is on the top, and richest on the bottom. The needle calibrates the fuel to the change in throttle valve (slide) opening. The further down the needle is, the later the main jet comes on and the leaner the mixture at that point.
4) Main Jet - This circuit affects the mixture from 1/2 throttle to full throttle. This is your full blast top end circuit. This circuit is most accurately tuned by checking the plug for the correct color, sort of mocha brown.
5) Float/Float valve/Float level - Your owner's manual has the correct level and procedure for calibrating your bike. Here are some symptoms that would indicate the need for float adjustment.
A) If float level is too high, the float bowl overflows out the drain hoses, and fuel often blocks the air passages that allow the carb to de-pressurize. This causes a bog, or hiccup over large hits.
B) If float level is too low, the engine starves for fuel off idle, causing a "boooooowang" sound very similar to a pilot jet or airscrew set too lean.
Every component overlaps the throttle range of one or more of the other components. Most people only change needle clip position, and pilot/main jets. Most people check jetting by looking at the plug or the exhaust spooge. We will attempt to jet by how the bike sounds and feels in response to throttle input at different rpm starting points. We won't really address the cutaway because of price. Not too many people can jet a bike right the first time; all jetting is trial and error, because every bike is different.
How to Jet:
1) Mark your throttle housing and grip in 1/4-turn increments. Use a marker, razor blade.
2) Now get the motor to operating temperature by riding around, away from the pits.
3) Once the motor is warm, ride in 2nd and 3rd gear from the low RPM to high RPM. This puts a good load on the motor and is an accurate test of performance.
4) Now try to locate the throttle opening at which the problem exists.
To develop your skill at jetting, you need to experience too rich and too lean with every jet (pilot, needle diameter, needle clip, and main jet). Only after you feel what too rich and too lean is will you be able to jet your bike perfectly. One size smaller in the main or dropping the needle a position or two can often make a huge difference in performance! Keep in mind that one area (i.e. throttle opening) can be rich while another is lean. For example, the midrange can be lean, which will cause a miss, but the top end can be too rich, which would cause blurbbering.
Step 1: Main Jet
The main jet controls the mixture at full throttle. It is possible to foul your plug if the main jet is too rich (but only if you're running at or almost at full throttle). Notice that we are talking about throttle openings here, not RPM. Other jets have negligible effect at full throttle.
Your objective is to get an understanding of the mixture at full throttle (wide open) operation. You need a long up-hill straightaway for this test so you can be in the top gear with the engine under load and running up in the upper RPM range.
If you hear pinging or missing, it is running lean; go larger on the main. If full throttle causes gasping and poor pulling at mid RPM’s, it is again an indication of running lean, so go larger on the main.
If the bike runs clean, select a larger main jet until you find the jet that causes a blurbbering (four-cycling) sound. When you experience that sound, you have found the jet that causes you to run too rich. So back off one size to a smaller jet. This is the safe main jet to use. You could go another size leaner; but you need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side.
Step 2: Jet Needle Clip Position and Needle Selection
The needle jet controls the mixture from 1/8 to 3/4 throttle. At the upper end of this range, the main plays a part too, but you have already selected the main jet. The slide cutaway and the needle DIAMETER are very similar in their actual effect on jetting. It's usually easier to leave the slide stock and try to get jetting to an optimum with the needle diameter. It is also possible to foul your plug if the jet needle setting causes the engine to run too rich. Raising the clip, a notch or two on most bikes will help substantially. Most bikes come from the factory jetted too rich, especially in the mid-range.
Again, you need an up-hill straight away to test the jet needle setting. This time use 1/2 throttle (it may be helpful to mark the openings) and allow the RPMs to reach the upper RPM range and leave the throttle there. Now do your listening.
Any pinging means too lean. Blurbbering means too rich. Excessive smoking is also an indication of running too rich.
To adjust to a richer mixture, take out the needle and place the clip one notch lower (nearer to the sharp end of the needle). This raises the needle out of the main jet a little higher, thus allowing more gas/oil mix to pass.
To adjust to a leaner mixture, take out the needle and place the clip one notch higher (further away from to the sharp end of the needle). This drops the needle to restrict the gas/oil mixture from flowing through the main jet.
Be careful when you remove the clip. It can flick away and become lost quite easily. I use a small flat screwdriver or a pair of needle-nose pliers and pry the clip away from the needle. I hold the needle and clip between my thumb and finger while I do this.
If you find that you are up at the top notch of the needle (there are usually 5 notches) you should get a needle with the next lower number (leaner) and place the clip in the center notch to give the equivalent mixture as the richer needle with the clip at the top. Similarly, if you find that you are up at the bottom notch of the needle you should get a needle with the next higher number (richer) and place the clip in the center notch to give the equivalent mixture as the leaner needle with the clip at the bottom.
You should not have to go to any more than one size leaner needle than stock. In addition, I would not expect you to need to go richer than the stock needle.
Next, go along slowly in one of the lower gears at less than 1/8 throttle and move the throttle quickly to 1/2 throttle. If the engine does not pull strongly, it just sort of gasps for breath and only runs well after the RPMs build up, it is an indication that the needle is too lean.
Step 3: Pilot Jet Selection and Air Correction Screw Adjustment
When you are at 1/4 throttle and more, the pilot jet has virtually NO effect. The pilot jet affects cranking, idle, and particularly acceleration from idle. If the pilot is too lean, the motor will hesitate when accelerating (in neutral) off idle. If it's too rich, the motor will smoke excessively, foul plugs, and be unresponsive off idle. After you get the needle right, get the bike idling by adjusting the idle stop up where it will idle slowly on its own. The air correction screw should be at 1.5 turns out to begin with. The airscrew affects the pilot somewhat; it's about like changing the pilot 1/2 step (if that were possible).
Okay, now slowly screw in the air correction screw 1/4 turn. Try to find the airscrew adjustment where you get maximum idle RPM. You may have to open the screw to 1.75 or 2.0 turns to get the right setting. As the idle RPM increases, turn down the idle stop to return the idle RPM to a slow, correct idle.
You objective is to find the pilot jet that will give you maximum idle with the airscrew set at 1.5 turns out.
Remember that opening the air correction screw admits more air during almost-closed throttle making a leaner idle mixture.
Now, when you find the correct jet size you will want to do some final air screw adjusting to improve throttle response, assuming that the needle jet is properly adjusted. Let the engine idle for 5 seconds then open the throttle abruptly and be aware of how the engine responds. If it almost dies, then you need a slightly richer idle mixture so that as that mixture is gulped at abrupt throttle openings it will be just a little rich and therefore give good response. The bottom end should be a little on the rich side, while the mid and upper end (the needle and main jet) should be leaner to give you the best throttle response.
You can test this as you ride, say slowly in 2nd gear, with the throttle closed so that the engine is drawing only from the pilot. Wick it open quickly. If you find yourself doing a wheelie, your air correction screw is set right. If instead the sound is uhhhhhhhhhwwaaaaaaa and you find your nose a few inches from your triple clamp, then your idle mixture is too lean and you need to close the screw a little.
Rich Jetting Symptoms and Solutions
A) Motor won’t idle and idle set screw is ineffective. Try leaning out the airscrew by turning it counter clockwise a quarter turn at a time. If the airscrew has no effect, install a leaner pilot jet and return the airscrew to 1 turn out. Proper airscrew settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the airscrew to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.
If you turn your airscrew 3 turns out you would need a smaller pilot.
B) Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle.
C. Motor will rev through the mid range then becomes gurgled (technical term) at full throttle and power is sluggish. This is a textbook example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine-tune the main jet with plug checks.
Plug checks are the key to fine tuning the main jet once the other circuits are set.
White Porcelain Lean
Chocolate Mocha Brown Perfect!
Black/Wet Rich
Look for symmetrical burn patterns, smell for odors like burnt plastic, which is actually transmission fluid and would be indicative of a blown seal.
Lean Jetting Symptoms and Solutions
A) Motor hesitates off idle with a "Booooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn’t come right back down to idle, install a richer pilot, and reset airscrew.
If you throttle on from idle and it bogs try turning the airscrew in or out a 1/4 of a turn at a time until it does not bog at all.
If it still bogs - turn it in. If you have to go in more than 1/2 or 3/4's of a turn in you'll need to put in a bigger pilot jet.
B) Motor knock knocks at idle when hot. Try solution A. If problem persists, perform a "leak down pressure test" to check for ignition crank seal leakage.
C) Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the trail and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize.
Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.
Name It Affects
1) Pilot jet 1/16 to 1/4 throttle
2) Air screw Idle to just off of idle (not the idle screw)
3) Jet needle 0-1/2 throttle, work with the needle shape first, then the clip position
4) Needle jet 1/8-3/4 throttle, also called nozzle
5) Slide cutaway 1/4-3/4 throttle, the angle of the cut affects air intake (high-dollar item)
6) Main jet 1/2-full throttle
1) Pilot jet - Affects mixture from idle to 1/3 throttle opening. The pilot jet meters fuel to the "bottom end circuits"
2) Air screw - Meters air to pilot jet. It is usually located near the back or air box side of the carb. Turning in clockwise will richen the pilot mixture. Turning out, counter clockwise, will lean out the pilot mixture. Average setting for most two strokes is 1 to 2 1/2 turns out. Refer to owner's manual. If the airscrew is set to less than 1 turn, it needs a bigger pilot.
3) Needle - Affects the mixture from 1/4 throttle to full throttle. The needle is in the leanest position when the clip is on the top, and richest on the bottom. The needle calibrates the fuel to the change in throttle valve (slide) opening. The further down the needle is, the later the main jet comes on and the leaner the mixture at that point.
4) Main Jet - This circuit affects the mixture from 1/2 throttle to full throttle. This is your full blast top end circuit. This circuit is most accurately tuned by checking the plug for the correct color, sort of mocha brown.
5) Float/Float valve/Float level - Your owner's manual has the correct level and procedure for calibrating your bike. Here are some symptoms that would indicate the need for float adjustment.
A) If float level is too high, the float bowl overflows out the drain hoses, and fuel often blocks the air passages that allow the carb to de-pressurize. This causes a bog, or hiccup over large hits.
B) If float level is too low, the engine starves for fuel off idle, causing a "boooooowang" sound very similar to a pilot jet or airscrew set too lean.
Every component overlaps the throttle range of one or more of the other components. Most people only change needle clip position, and pilot/main jets. Most people check jetting by looking at the plug or the exhaust spooge. We will attempt to jet by how the bike sounds and feels in response to throttle input at different rpm starting points. We won't really address the cutaway because of price. Not too many people can jet a bike right the first time; all jetting is trial and error, because every bike is different.
How to Jet:
1) Mark your throttle housing and grip in 1/4-turn increments. Use a marker, razor blade.
2) Now get the motor to operating temperature by riding around, away from the pits.
3) Once the motor is warm, ride in 2nd and 3rd gear from the low RPM to high RPM. This puts a good load on the motor and is an accurate test of performance.
4) Now try to locate the throttle opening at which the problem exists.
To develop your skill at jetting, you need to experience too rich and too lean with every jet (pilot, needle diameter, needle clip, and main jet). Only after you feel what too rich and too lean is will you be able to jet your bike perfectly. One size smaller in the main or dropping the needle a position or two can often make a huge difference in performance! Keep in mind that one area (i.e. throttle opening) can be rich while another is lean. For example, the midrange can be lean, which will cause a miss, but the top end can be too rich, which would cause blurbbering.
Step 1: Main Jet
The main jet controls the mixture at full throttle. It is possible to foul your plug if the main jet is too rich (but only if you're running at or almost at full throttle). Notice that we are talking about throttle openings here, not RPM. Other jets have negligible effect at full throttle.
Your objective is to get an understanding of the mixture at full throttle (wide open) operation. You need a long up-hill straightaway for this test so you can be in the top gear with the engine under load and running up in the upper RPM range.
If you hear pinging or missing, it is running lean; go larger on the main. If full throttle causes gasping and poor pulling at mid RPM’s, it is again an indication of running lean, so go larger on the main.
If the bike runs clean, select a larger main jet until you find the jet that causes a blurbbering (four-cycling) sound. When you experience that sound, you have found the jet that causes you to run too rich. So back off one size to a smaller jet. This is the safe main jet to use. You could go another size leaner; but you need to be careful to avoid running too lean which causes the engine to run hot and could seize the piston. It's better to jet on the rich side.
Step 2: Jet Needle Clip Position and Needle Selection
The needle jet controls the mixture from 1/8 to 3/4 throttle. At the upper end of this range, the main plays a part too, but you have already selected the main jet. The slide cutaway and the needle DIAMETER are very similar in their actual effect on jetting. It's usually easier to leave the slide stock and try to get jetting to an optimum with the needle diameter. It is also possible to foul your plug if the jet needle setting causes the engine to run too rich. Raising the clip, a notch or two on most bikes will help substantially. Most bikes come from the factory jetted too rich, especially in the mid-range.
Again, you need an up-hill straight away to test the jet needle setting. This time use 1/2 throttle (it may be helpful to mark the openings) and allow the RPMs to reach the upper RPM range and leave the throttle there. Now do your listening.
Any pinging means too lean. Blurbbering means too rich. Excessive smoking is also an indication of running too rich.
To adjust to a richer mixture, take out the needle and place the clip one notch lower (nearer to the sharp end of the needle). This raises the needle out of the main jet a little higher, thus allowing more gas/oil mix to pass.
To adjust to a leaner mixture, take out the needle and place the clip one notch higher (further away from to the sharp end of the needle). This drops the needle to restrict the gas/oil mixture from flowing through the main jet.
Be careful when you remove the clip. It can flick away and become lost quite easily. I use a small flat screwdriver or a pair of needle-nose pliers and pry the clip away from the needle. I hold the needle and clip between my thumb and finger while I do this.
If you find that you are up at the top notch of the needle (there are usually 5 notches) you should get a needle with the next lower number (leaner) and place the clip in the center notch to give the equivalent mixture as the richer needle with the clip at the top. Similarly, if you find that you are up at the bottom notch of the needle you should get a needle with the next higher number (richer) and place the clip in the center notch to give the equivalent mixture as the leaner needle with the clip at the bottom.
You should not have to go to any more than one size leaner needle than stock. In addition, I would not expect you to need to go richer than the stock needle.
Next, go along slowly in one of the lower gears at less than 1/8 throttle and move the throttle quickly to 1/2 throttle. If the engine does not pull strongly, it just sort of gasps for breath and only runs well after the RPMs build up, it is an indication that the needle is too lean.
Step 3: Pilot Jet Selection and Air Correction Screw Adjustment
When you are at 1/4 throttle and more, the pilot jet has virtually NO effect. The pilot jet affects cranking, idle, and particularly acceleration from idle. If the pilot is too lean, the motor will hesitate when accelerating (in neutral) off idle. If it's too rich, the motor will smoke excessively, foul plugs, and be unresponsive off idle. After you get the needle right, get the bike idling by adjusting the idle stop up where it will idle slowly on its own. The air correction screw should be at 1.5 turns out to begin with. The airscrew affects the pilot somewhat; it's about like changing the pilot 1/2 step (if that were possible).
Okay, now slowly screw in the air correction screw 1/4 turn. Try to find the airscrew adjustment where you get maximum idle RPM. You may have to open the screw to 1.75 or 2.0 turns to get the right setting. As the idle RPM increases, turn down the idle stop to return the idle RPM to a slow, correct idle.
You objective is to find the pilot jet that will give you maximum idle with the airscrew set at 1.5 turns out.
Remember that opening the air correction screw admits more air during almost-closed throttle making a leaner idle mixture.
Now, when you find the correct jet size you will want to do some final air screw adjusting to improve throttle response, assuming that the needle jet is properly adjusted. Let the engine idle for 5 seconds then open the throttle abruptly and be aware of how the engine responds. If it almost dies, then you need a slightly richer idle mixture so that as that mixture is gulped at abrupt throttle openings it will be just a little rich and therefore give good response. The bottom end should be a little on the rich side, while the mid and upper end (the needle and main jet) should be leaner to give you the best throttle response.
You can test this as you ride, say slowly in 2nd gear, with the throttle closed so that the engine is drawing only from the pilot. Wick it open quickly. If you find yourself doing a wheelie, your air correction screw is set right. If instead the sound is uhhhhhhhhhwwaaaaaaa and you find your nose a few inches from your triple clamp, then your idle mixture is too lean and you need to close the screw a little.
Rich Jetting Symptoms and Solutions
A) Motor won’t idle and idle set screw is ineffective. Try leaning out the airscrew by turning it counter clockwise a quarter turn at a time. If the airscrew has no effect, install a leaner pilot jet and return the airscrew to 1 turn out. Proper airscrew settings are usually between 1 and 2 1/2 turns out. The correct pilot for your bike is one that allows instant off-idle throttle response, and allows the airscrew to be effective between 1 and 2.5 turns out. If the problem still exists, check and clean the choke circuit and carb vent lines.
If you turn your airscrew 3 turns out you would need a smaller pilot.
B) Motor stumbles off idle to 1/2 throttle, then cleans up. If everything in part A above is correct, check the needle. To lean out the needle, you need to raise the clip. If the needle is dropped all the way lean and problem still exists, try a leaner needle.
C. Motor will rev through the mid range then becomes gurgled (technical term) at full throttle and power is sluggish. This is a textbook example of a rich main jet. This problem often occurs when an air filter becomes clogged and gets worse as the motor heats up. Choose a main jet with a smaller number and lean out the air fuel mixture one step at a time. If the problem persists even though it’s improved a little, reinstall your original main jet and lean out the needle one position. Now fine-tune the main jet with plug checks.
Plug checks are the key to fine tuning the main jet once the other circuits are set.
White Porcelain Lean
Chocolate Mocha Brown Perfect!
Black/Wet Rich
Look for symmetrical burn patterns, smell for odors like burnt plastic, which is actually transmission fluid and would be indicative of a blown seal.
Lean Jetting Symptoms and Solutions
A) Motor hesitates off idle with a "Booooowang" sound. This is a lean symptom and often occurs when a motor is cold. Try turning the air-fuel screw clockwise a quarter turn richer and check throttle response again until motor revs without hesitation. If the motor "hangs up" or doesn’t come right back down to idle, install a richer pilot, and reset airscrew.
If you throttle on from idle and it bogs try turning the airscrew in or out a 1/4 of a turn at a time until it does not bog at all.
If it still bogs - turn it in. If you have to go in more than 1/2 or 3/4's of a turn in you'll need to put in a bigger pilot jet.
B) Motor knock knocks at idle when hot. Try solution A. If problem persists, perform a "leak down pressure test" to check for ignition crank seal leakage.
C) Motor revs clean and crisp but runs really hot and lacks power. This symptom indicates a lean main jet and/or needle. Use a richer main jet and/or needle setting. This is the most common misconception about two stroke jetting: When a motor runs excessively hot and lacks power, it is on the verge of seizing. If you are on the trail and don’t have the right main jet, try raising the needle by lowering the clip. It won’t be exact but at least you won’t seize.
Note: To be certain that your problem is lean, pull on the choke and see if the problem gets better or worse. If better, your problem is a lean condition.
