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How to "feel" when running too rich/lean?

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I'm used to throwing a car on the dyno with a wideband to get the air/fuel in the right range. I know, although not the safest, leaner typically means more power up to a certain point. Oil being in the fuel has to change things quite a bit. How are you guys dialing in air/fuel ratios off the butt dyno?

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Most accurate is a plug chop. Next a plug read (color). The butt dyno is not real accurate. If the bike is running too rich it feels boggy on top and fouls plugs. Too lean feels fluttery and weak. Hopefully you don't run it this way for long, terminal for engines. 

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It does take a certain amount of experience

Rich='Wet', "heavy/thick" Sounds/sensation

Lean="Dry", "Light/thin" Sounds/sensation

 

You can also do tests with a stop watch, a mild hill long enough to make a decent ride on.

 

In third/fourth gear, ride up the hill at steady throttle. Mark the grip so you know where you are in relation to ho open the arb is (1/8 incements from closed to fully open is enough). Time the runs.

 

For example, say you do it 3/8ths throttle. You will be 'on the needle', transitioning from straight to taper. Moving the clip and or different straight diameters will change the time it takes, at the same throttle position to 'make the run'. Your goal is to make the run, at the steady throttle position, in the shortest time. You will find that a few needles or clip changes do not seem to do anything to your time. leave the jetting on the rich side. Your run should be at least thirty seconds long to remove stopwatch user error.

 

Do this for every 1/8th of a turn of the throttle. Keep good notes. Incluse baromentric pressure, temp and humdity (components of air density). If these change, you may have to change jetting to suit.

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Wow. I complete grasp the concept although I think the human factor may introduce far too many variables to keep things accurate. A buddy of mine has a small 4 wheeler dyno, maybe I'll try that. Way quicker and way more accurate. Thank you for the thorough responses.

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Wow. I complete grasp the concept although I think the human factor may introduce far too many variables to keep things accurate. A buddy of mine has a small 4 wheeler dyno, maybe I'll try that. Way quicker and way more accurate. Thank you for the thorough responses.

It's not as hard as it sounds, and jetting can be changed in 30 minutes. I bought my first bike 6 months ago, and a member on here was kind enough to PM me and walk me through the process. It taught me a lot about how the carb works, and then I became infatuated with jets and needles. It can be done off the back of the truck, in between trails, and when you find that magic combination you are in heaven. The main thing is to change one thing at a time. Like William1 said, find out where in the throttle your problem is, and start there. The pilot solved 90% of my issues, the needle refined it, and I'm old and slow so I just assume the main is right :facepalm:

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sparkplug.gif

 

 

 

 

Spanky's jetting 101:

 
A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. 
 
A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. Hard starting when hot or cold, poor response when opening the throttle, reluctance to idle, all of these are symptoms of an improperly sized pilot jet or incorrectly adjusted air screw. 
 
The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. 
 
A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift. 
 
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, leading to deposits and wet fouling of the plugs. 
 
Do you have spooge? There are the rare instances where a mechanical issue, such as a leaking wet-side crank seal, can cause spooge. But, by and large, this isn’t the case. In most instances, spooge is caused by rich jetting. It has nothing to do with how much oil you mix in the gas, or how hard you ride. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, resulting in deposits, plug fouling, and spooge. Spooge is nothing more than unburned fuel and oil entering the exhaust. 
 
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Someone with a good understanding of jetting can get you in the ball park, but you need to do the testing to determine the correct jetting yourself if you want it right. 
 
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best. 
 
It's very important that you start with the pilot circuit, because the pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving. 
 
Before you start to re-jet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. 
 
One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting. 
 
Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect yourjetting all across the throttle range. 
 
All jet testing must be done with the engine at full operating temperature. 
 
As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response. 
 
Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. If your engine doesn’t respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet. 
 
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. 
 
The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day. 
 
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply. 
 
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan. 
 
Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isn’t one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting. 
 
The slide is also a tuning variable for jetting, affecting the throttle range from 1/8 throttle to approximately 1/3 throttle. If you can’t clean up the small-throttle jetting on your bike no matter how lean you go with the pilot or the needle, the slide is the next step. But few bikes need leaner slides. 
 
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. 
 
Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. Their purpose is the same, they just do it in different ways.
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Big thanks for that. I've been a gear head for years but never messed with a carb, ever. It's probably backwards, but efi seems so much easier and far more accurate. Sounds like a lot of this carb tuning goes off how a bike feels and sounds and my ear is quite ignorant at this point, so it will be a learning process. I look forward to it tho. I'm 35 and my body is already fairly beat up from sports, but I love the tinkering and my son has fallen head over heels with riding. I very much look forward to learning and teaching. Thanks again.

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sparkplug.gif

 

 

 

 

Spanky's jetting 101:

 
A correctly jetted carb makes a tremendous difference in the torque, midrange pull, top-end pull, and over-rev of your engine. If you have never jetted your bike correctly, you will almost certainly gain some performance at some point in the bike's powerband. 
 
A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. Hard starting when hot or cold, poor response when opening the throttle, reluctance to idle, all of these are symptoms of an improperly sized pilot jet or incorrectly adjusted air screw. 
 
The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. 
 
A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift. 
 
Are you fouling plugs? Many people will tell you all sorts of band-aid fixes, from running less oil, to running a hotter plug. Both are incorrect fixes for plug fouling. It's all in the jetting. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, leading to deposits and wet fouling of the plugs. 
 
Do you have spooge? There are the rare instances where a mechanical issue, such as a leaking wet-side crank seal, can cause spooge. But, by and large, this isn’t the case. In most instances, spooge is caused by rich jetting. It has nothing to do with how much oil you mix in the gas, or how hard you ride. An engine that is jetted too rich will have combustion temperatures that are too low to burn the fuel and oil effectively, resulting in deposits, plug fouling, and spooge. Spooge is nothing more than unburned fuel and oil entering the exhaust. 
 
The only way to know what jetting changes you will need is by trial-and-error. No one can give you jetting specs, because every bike is different, every rider has a different style, and jetting is totally weather dependent. Unless the person telling you what jets to use is riding an identical bike, on the exact same track, at the same time, his recommendations are meaningless. Someone with a good understanding of jetting can get you in the ball park, but you need to do the testing to determine the correct jetting yourself if you want it right. 
 
Jetting is fairly simple, and is a useful skill to learn if you ride a two-stroke and want it to perform at it's best. 
 
It's very important that you start with the pilot circuit, because the pilot circuit affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel as well, adding to the total amount of fuel that your engine is receiving. 
 
Before you start to re-jet your bike, you need a clean air filter, a fresh plug (actually you need several plugs to do plug-chop tests for the main jet), and fresh fuel. 
 
One important detail: Make sure the engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Trying to jet a worn out engine is a waste of time. The same goes for reeds that don't seal properly, and a silencer that needs re-packing. Worn reeds will mimic rich jetting, and worn rings will mimic lean jetting. 
 
Before you start the jet testing, Install a fresh plug. Set the float level to the proper specs, an incorrect float height will affect yourjetting all across the throttle range. 
 
All jet testing must be done with the engine at full operating temperature. 
 
As already stated, start with the pilot circuit. Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idling. Turn the air screw slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response. 
 
Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. If your engine doesn’t respond to air screw changes, then you either have a dirty carb, or the pilot jet is way too rich. When the pilot jet is way too rich, you are forced to use the idle screw to open the slide so far in order to keep the engine running that the pilot circuit is partially bypassed, and the engine is actually starting to draw fuel through the needle jet. 
 
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the air screw for the fastest idle, it's time to tune the air screw for the best throttle response. Again, make sure the bike is at full operating temperature. Set the idle back down (the bike should still idle, despite what you read in the Moto Tabloids), and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. 
 
The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing outdoor temps and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the heat of the mid-day. 
 
Now, it's time to work on the needle. Mark the throttle grip at 1/4 and 3/4 openings. Ride the bike between these two marks. If the bike bogs for a second before responding to throttle, lower the clip (raising the needle) a notch at a time until the engine picks up smoothly. If the bike sputters or sounds rough when giving it throttle, raise the clip (lowering the needle) until it runs cleanly. There isn't really any way to test the needle other than by feel, but it's usually quite obvious when it's right or wrong. A too-rich needle can often be felt simply when revving the bike on the stand. The bike will sound rough and raspy when blipping the throttle on the stand. A correctly jetted bike should rev cleanly and crisply. 
 
Last is the main jet. The main jet affects from 1/2 to full throttle. The easiest way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight, through all gears. As soon as the bike tops out, pull the clutch in, and kill the engine, coasting to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan. 
 
Please note that, when reading plugs, the tip of the insulator, threads, etc. are meaningless for jetting purposes. They can tell you a lot of things, but jetting isn’t one of them. Only the mixture ring at the very base of the insulator, inside the threads, can tell you anything about the jetting. 
 
The slide is also a tuning variable for jetting, affecting the throttle range from 1/8 throttle to approximately 1/3 throttle. If you can’t clean up the small-throttle jetting on your bike no matter how lean you go with the pilot or the needle, the slide is the next step. But few bikes need leaner slides. 
 
Keep in mind, even though this article is intended primarily for two-strokes, four-strokes also need proper jetting to perform right, although they are not quite as fussy as their oil-burning cousins. The only real difference in the two is with the pilot circuit. 
 
Two-strokes have an air screw that you screw in to make the jetting richer, and screw out to make the jetting leaner. Four-strokes, on the other hand, have a fuel adjustment screw that you screw in to make the jetting leaner, and out to make it richer. Their purpose is the same, they just do it in different ways.

 

This is a great read for carb tuning , i have a page saved like this but this actually explains it in ways easier to understand . The pre-school pictures awesome too  :lol:

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Thanks. :D

This topic comes up so often that I sat down a long time ago and wrote this up do I could just copy/paste it in jetting threads. I had no idea just how far it would spread though, if you do a Google search on Spanky's Jetting this article is all over the net in multiple forums. I should have copyrighted it...:lol:

Edited by Chokey
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Thanks. :D

This topic comes up so often that I sat down a long time ago and wrote this up do I could just copy/paste it in jetting threads. I had no idea just how far it would spread though, if you do a Google search on Spanky's Jetting this article is all over the net in multiple forums. I should have copyrighted it...:lol:

Copyright, then print & distribute :lol: , everyone could benefit from one in their tool box

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Thanks. :D

This topic comes up so often that I sat down a long time ago and wrote this up do I could just copy/paste it in jetting threads. I had no idea just how far it would spread though, if you do a Google search on Spanky's Jetting this article is all over the net in multiple forums. I should have copyrighted it...:lol:

I didn't even notice, realize you wrote it. I haven't had the chance to look into the actual jetting yet, but I did go through the proper setting of the air screw and I have certainly found more power. I had actually felt a bit of power loss, groggy, tiny bog going on and this remedied it immediately. I was thinking the bike was rich and adding to the air screw. By the time I was done setting and checking throttle response, I was only 1 turn out from seat. This is borderline needing to jet down, correct?

Regardless, thanks a ton for your time in writing that up. It helped with my understanding as well as putting some spun back in my life. ;-)

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I didn't even notice, realize you wrote it. I haven't had the chance to look into the actual jetting yet, but I did go through the proper setting of the air screw and I have certainly found more power. I had actually felt a bit of power loss, groggy, tiny bog going on and this remedied it immediately. I was thinking the bike was rich and adding to the air screw. By the time I was done setting and checking throttle response, I was only 1 turn out from seat. This is borderline needing to jet down, correct?

 

 

Not necessarily. The air screw can actually be effective down to about  half turn out. I just use the 1 to 2.5 turn "rule" because it makes it easier to explain. And I try to jet my bike so that it' at it's most responsive at about 1.5 turns out, but different people have different preferences. A lot of riders prefer to be nearer the 1 turn mark.

 

If you're satisfied with how it's running, that's all that really matters.

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