Jetting!

[Dell30rb]

Everyone needs to know about jetting! And there are many different methods. Here is my method. (I got this info from JMD at woodsracer, so I have to give him some credit)


I don't read the plug, except for fine-tuning. Here's a thumbnail sketch for getting it pretty close by feel. Find a long, straight stretch to test. First work on the main jet. It works mostly on wide-open throttle. Go wide open up the straight. If your bike blubbers and misses, you're too rich. If it runs clean but seems to run out of power, you're too lean. (Almost every bike in the world is too rich; don't ask me why, but it's true. Probably because the too-lean bikes have already blown up.) Lean the main jet one step at a time until it stops blubbering and runs clean. Then go leaner by steps until it seems to run too lean. One step richer than that is correct. Next work on the needle (1/8 throttle to 3/4 throttle). Run up the straight again at half throttle. Same deal. If it blubbers, raise the clip on the needle (lowering the needle into the needle jet and leaning the mixture) until it cleans up. If it's too lean, lower the clip. If you run out of clip positions, you'll have to buy a new needle. Try Sudco for good prices on jets of all kinds.

Then check the pilot (closed throttle to 1/8). If the bike starts without the choke, it's on the lean side, but that's not necessarily bad. Start the bike, warm it up, and let it idle. Then blip the throttle. If the revs hang up there a while before coming down, it's too lean. If the revs fall quickly and the bike dies, its too rich. Once that's set properly, adjust your air screw to get a proper idle speed (counterclockwise leans it and increases the idle, and vice-versa). If you find that your air screw must be more than two turns out (from completely in), you should get a leaner pilot jet. If the air screw must be less than a half- or quarter-turn out, you need a richer pilot jet.

Keep in mind that each circuit (main, needle, pilot) affects all the others slightly. So when you're done with your needle and pilot adjustments, you may want to check the main jet again. But you should be able to get it pretty close with this method.

Lets hear it!

[Rockey]

Hey Ren,
I'd just like to reccomend after all is finished with the main, do a WOT plug chop just to confirm you are not too lean. That nice smooth power delivery cna be deciving, as I learned the hard way. Great Guide Man!

[mx757]

i just kinda wing it. here in colorado the weather changes so much that if i worried about jetting all the time id never get to ride. plus in our local series we move elevations so much it sucks. we go from 5000 feet to 7200. i just think that as long as im not fouling plugs and my bikes revving right its fine.

[Dell30rb]

You just gotta watch out for that excessive leanness. If you are climbing a big hill, wide open for a long period of time, and your jetting is just a smidge lean, you might be lookin at a seizure!

[mx757]

yeah we usually run it just a bit rich but dont really do much. just make sure its close.

[Dell30rb]

any more comments? I am thinking about moving this to the KB forum, but I just want to see if it can arouse some more conversation before it is moved.

[jmiakaike]

Originally posted by dell30rb
any more comments? I am thinking about moving this to the KB forum, but I just want to see if it can arouse some more conversation before it is moved.

dell,
look around here under the title, 'if you have money to burn' is a post i made... electronic fuel injection for the yz426 and crf450s... we don't need no stinkin jets!!>..lol

james


(ok, that is at the very BOTTOM of my wish list $3000 for it, costs more than my bike did/will..., unless i win the lottery, than it goes right to #3... behind the new street bike and dirt bike and car and truck.. ok #5 then...)

[egoallenpoe393]

Jetting is an art but it does not take a phd to figure it out.

The advise on the first post is real good.

You definetly need a starting point. Plug reading is good if your racing and need to see what the engine is doing at the speed your runing, day you ride and so on. everything effects jetting from altitude, weather humity and air temp.

So plug reading is a good thing.

Jetting is what will give you a good plug read .

I dont undertand jetting dynamics, I only understand my experiance and what I have been taight in the past.

[Dell30rb]

Those new C-dales are fuel injected... but instead of being re-jetted, they need to be re-mapped.

[whyzee]

Ren, I've posted this in the past and thought I'd share it for those here, BTW, thanks for passing this place on to me. Good to read your posts again.
Dave


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).


I will post the rest next, I'm getting an error message in refrence to length.

[whyzee]

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. Stock position is usually in the middle, but check your owner’s manual to be sure. 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.

This is information I have gathered for some time now,I want to express that there are several authors involved in this and that I in no way express this information as being my own.

[Dell30rb]

Well, you sure outdid me here! Would you mind stopping by and jetting my bike... please...

[Florida 393]

WhyZee,
Welcome to Moto Emporium. That is an impressive bit of technical information. I hope you continue to post here and make this your moto home.


Bill

[jmiakaike]

Originally posted by 96whyzee125
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. Stock position is usually in the middle, but check your owner’s manual to be sure. 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.

This is information I have gathered for some time now,I want to express that there are several authors involved in this and that I in no way express this information as being my own.

96whyzee!

nice pics. i ilke the way you started it all with the ups truck! my ups driver even knows my dogs names!...lol

i should take pics of my mess, but then i would be embarrased of my messy area...lol

will post a pic of the beast when it is done though...


James

[jmiakaike]

96whyzee

nice pics!

i like the way you started it all with the UPS truck! i know how that is.. my ups driver knows the names of my dogs!...lol


i will post a pic of my beast when it is done..

welcome aboard as well!!


James

[Faded]

The float needs to be set first and foremost...

[ossagp]

Two things I disagree with on the two instructionals posted here. In the first one, I believe it is just an error in wording, "if your bike starts easily without the choke (few two strokes actually have a true "choke", but an enrichening circuit instead) it is too lean". Just the opposite is true on the starting.

In the second one, paragraph 3 states that the low speed circuit has little to no effect after 1/4 throttle: At about 1/8 to 1/4 throttle the low speed jet or circuit will be contributing as much fuel as it is going to, but it DOES NOT shutoff. As circuits are opened or exposed they stay open and or exposed until the throttle is closed past their point of actuation. As long as the throttle is open and you have vacuum (ie the motor is turning over) the circuits keep flowing fuel.

[YZ@43]

:I have read all the strings on jetting and have tried to find my gremlin. I have a 02 YZ250, I bought serveral weeks ago. I have changed the plug cleaned the air filter and box. all new fluids, and repacked the exhaste. The bike runs great as long as it is wide open. when you let off and down shift the bike starts to sputter. Any Help

[low430]

Originally Posted by YZ@43

:I have read all the strings on jetting and have tried to find my gremlin. I have a 02 YZ250, I bought serveral weeks ago. I have changed the plug cleaned the air filter and box. all new fluids, and repacked the exhaste. The bike runs great as long as it is wide open. when you let off and down shift the bike starts to sputter. Any Help

rich pilot jet?
Try leaning the air screw or going to a leaner pilot jet

[cwa11is]

The function of the throttle position / jet function described here concur with everything I have always believed, however, I was always taught and believed to start the testing and adjustment in the reverse order. Start with the air screw, pilot jet, needle setting, main jet. I'm not claiming to be a carb guru, but it just makes more sense to me to start at the bottom end.