EthansDad
06-08-2011, 08:47 PM
Hey All,
Been researching Jetting for about the last year now and wanted to share some ideas learned about jetting. The biggest lesson I've learned is its hard, and I'm no expert. also, there is no easy way to Jet besides understanding what is going on in that carb, what are your tuning goals, and how to test such that you get repeatable results. This guide is just a reference for all of that. hope it helps you jet better, or at least answer some questions about what's going on in that carb!
Theory of Jetting:
the main job of a carb is to mix air and fuel into a optimal ratio and feed it to the motor. simple, eh? fuel is metered into the motor by a series of jets with different size openings. higher numbers being a bigger opening will deliver more fuel. you can say the throttle position is only a suggestion of how much fuel is to be delivered to the motor, the job is ultimately carried out by the jet size.
the science going on is called stoichiometry (http://en.wikipedia.org/wiki/Stoichiometry). can read up on it in the link, but the short version is for any fuel and oxygen mix, there is one and only one optimum ratio which produces the maximum "boom". what that means is your motor only makes max power when your jetting is spot on delivering 14 parts oxygen to 1 part fuel, or 14:1. if I deliver less oxygen per unit of fuel, I am rich since the mixture now has more fuel per volume that before. the opposite is also true, if I have too much oxygen per volume, I am lean.
The fuel delivery is fairly straight forward and something that you as a tuner can control. its important to know that fuel is also a cooling agent to the motor. that is why if you run too lean, you burn your motor. the air is a different story. its oxygen that mixes with the gas to create the 'bang" needed for combustion. The problem is per any given volume of "air", you really don't know how much oxygen you are getting.
What is really needed to know is the air density. Let's say I grab a handful of "air" and call that exactly one unit of air. If I measured the make up, I'd find its 20% oxygen in that one unit. the problem is how densely packed is that unit of air I have in my hands? if the air is very dense, in the same handful, I might have 1.5 units of air, and now have a volume that contains 30% oxygen. That is going to change my air/fuel ratio since now per every intake period, I introduce more oxygen into the motor. that means the same jetting that ran great last time is now running lean, since there is more oxygen per intake period.
Things that effect air density are temperature, altitude, barometric pressure, and to some degree humidity.
Temp - the hotter it gets, the more excited the air gets atoms get, and therefore are more spread out per volume. so when its hot, there is less oxygen per volume, and of course when its colder, there is more oxygen per volume.
altitude - air has weight and our atmosphere is always pushing down on us. that causes density. when I am higher up, there is less air above me to the top of our atmoshpere, therefore there is less density. so, the higher I go, the less oxygen per volume. common sense there if you have ever been in the mountains, hard to breath!
barometric pressure - storm front and other weather events have an effect on air density. the higher the pressure, the more air per volume.
Humidity - from what I can read, it has some effect, but very little compared to the other 3. basically, if there is water in the air, it takes up space, so per volume of air, there is less oxygen since some of that space is taken up by water. so, if its humid, air density goes down.
Netting all of that above out in simple up/down relationship (AD=air density):
Read the below chart like this:
*the condition = how it effects air density = the new condition it puts on your existing jetting -->which way to change your jetting to correct
*Temp hotter = AD down = richer -->lower jet number
*Temp colder = AD UP =leaner = higher jet number
*altitude higher = AD down = richer -->lower jet number
*altitude lower to sea level = AD UP = leaner-->higher jet number
*bar press Down (low pressure system) = AD Down = richer -->lower jet number
*Bar pressure up(high pressure system) = AD UP = leaner -->higher jet number
*Very Humid = AD Down = richer --> lower jet number
*very dry = AD normal = n/a -->no jet change
a good rule of thumb, for any on event in one direction, I take a guess of 2 jet sizes, or points, so I might move from a 95 main to a 98 (next hop up) for one event. if I get two of these events going in the same direction (its colder and this track is closer to sea level for example), then 4 points. if one event is going in one direction, and another event in the opposite, well they cancel each other out!
Also, for any 4 point move on the main jet in one direction, its a good idea to move your needle clip one position in that same direction.
That rule of thumb has worked for me to get it in the ballpark, but sometimes, when all else fails, revert to the tuning guide and make sure your guesses on jet directions are correct!
A common mistake for a rookie 2-stroke tuner is to confuse the terms lean/rich as it relates to air/fuel to the oil/gas ratio. it can get confusing. I used to think those two where totally different, but came across a read that shows they are related. remember how with humidity the water in the air takes up 'space' so it effects air density? well, when you put oil in gas, it also takes up space in your fuel. so for every one unit of fuel that has oil in it, there is less actual fuel! sounds picky, but seems like the professional 2 stroke mechs in the past used to fine tune air/fuel ratios track side by adjusting their 2 stroke oil/fuel ratio. if they needed to be say 1 jet size leaner, or even a half jet size, they would compensate by taking away more fuel per volume by adding more oil to the gas! tricky!
Goals of jetting
produce max power at all times. simple, yes? another way to say that is I want to keep an ideal ratio for every throttle position. for the sake of keeping it simple, let's say I just have one jet in my carb. What you need is to find out the air density and change your jet such that the ratio is 14:1 in any conditions.
Jetting howto
there are already some good guides out there that I'll link to. I don't think I can write up anything better than these.
Guide 1 (http://www.kawasakimotorcycle.org/forum/kawasaki-motocross-offroad/17227-spankeys-jetting-guide-info-2-stroke-riders-must-have.html)
More Detailed Guide (http://www.eric-gorr.com/pdfs/carb%20cleaning%20and%20jetting.pdf)
What I will add is it probably won't work for you the first time. don't quit, keep at it! its a good idea to document your jetting. that way, if you get too far off center, you can always go back to a starting place. make small moves in one direction, and usually only change one circuit at a time. I personally like to start with the pilot jet since it effects the entire range, unlike any other jet.
Here is how I document jetting:
Carb size (like 20, 21mm, etc)
main jet
pilot jet
needle position
current temp
current altitude (for different tracks)
air screw position
What motor config
Which pipe
Advanced Tools and testing
If I had a gauge on the carb the gave me real time readings on fuel to oxygen readings, it would be easy to jet. the problem is you never know that info. there are, however a few tools and tricks that can help you get your ratio right on air/fuel.
RAD gauge - or relative air density gauge. What a RAD gauge does is put all the big factors of air density into one common scale of percentage. so temp, altitude, and barometric pressure can all be netted out to tell me if I need to go up or down in jetting for any given condition. What the RAD gauge does is tell when when any of these factors go up or down enough to effect overall air density. so for example, I can be sitting trackside and see that the temp has gone up 10deg, but there is also a storm front coming in -the RAD gauge nets it all out and tells me air density has gone down 10%, and I need to go down 10% on jet size to keep my ratio correct. The down side to a RAD gauge is it won't help you jet the bike the first time. you need another method to get it spot on the first time, then going forward you can use the rad gauge to keep that ratio balanced for any given air condition.
EGT - exhaust gas temp. a neat by product of Stoichiometry is that different ratios of air:gas affect the temperature of the exhaust gas. the less fuel, the hotter, the more fuel (richer) the cooler the exhaust gasses will be. I've found this to be an effective method to get jetting dialed in for the first time. I'm not 100% sure on the ideal temp, but my best guess is that you want the exhaust to burn at 800 deg Fahrenheit under load. If I am burning at 750F, I am rich and losing power and if I am burning at 900F, I am likely about to loose my motor. for reference, in a car the ideal temp is 1200F. aluminum melts at 1200F, but the cylinders never get hot enough since the spent fuel (hot gas) is expelled so fast.
the good parts of EGT it is measurable and exact and very valuable for first time jetting or absolute checks. the down side, you need to modify your pipe to run one and also get some testing equipment to gather the probe data.
Been researching Jetting for about the last year now and wanted to share some ideas learned about jetting. The biggest lesson I've learned is its hard, and I'm no expert. also, there is no easy way to Jet besides understanding what is going on in that carb, what are your tuning goals, and how to test such that you get repeatable results. This guide is just a reference for all of that. hope it helps you jet better, or at least answer some questions about what's going on in that carb!
Theory of Jetting:
the main job of a carb is to mix air and fuel into a optimal ratio and feed it to the motor. simple, eh? fuel is metered into the motor by a series of jets with different size openings. higher numbers being a bigger opening will deliver more fuel. you can say the throttle position is only a suggestion of how much fuel is to be delivered to the motor, the job is ultimately carried out by the jet size.
the science going on is called stoichiometry (http://en.wikipedia.org/wiki/Stoichiometry). can read up on it in the link, but the short version is for any fuel and oxygen mix, there is one and only one optimum ratio which produces the maximum "boom". what that means is your motor only makes max power when your jetting is spot on delivering 14 parts oxygen to 1 part fuel, or 14:1. if I deliver less oxygen per unit of fuel, I am rich since the mixture now has more fuel per volume that before. the opposite is also true, if I have too much oxygen per volume, I am lean.
The fuel delivery is fairly straight forward and something that you as a tuner can control. its important to know that fuel is also a cooling agent to the motor. that is why if you run too lean, you burn your motor. the air is a different story. its oxygen that mixes with the gas to create the 'bang" needed for combustion. The problem is per any given volume of "air", you really don't know how much oxygen you are getting.
What is really needed to know is the air density. Let's say I grab a handful of "air" and call that exactly one unit of air. If I measured the make up, I'd find its 20% oxygen in that one unit. the problem is how densely packed is that unit of air I have in my hands? if the air is very dense, in the same handful, I might have 1.5 units of air, and now have a volume that contains 30% oxygen. That is going to change my air/fuel ratio since now per every intake period, I introduce more oxygen into the motor. that means the same jetting that ran great last time is now running lean, since there is more oxygen per intake period.
Things that effect air density are temperature, altitude, barometric pressure, and to some degree humidity.
Temp - the hotter it gets, the more excited the air gets atoms get, and therefore are more spread out per volume. so when its hot, there is less oxygen per volume, and of course when its colder, there is more oxygen per volume.
altitude - air has weight and our atmosphere is always pushing down on us. that causes density. when I am higher up, there is less air above me to the top of our atmoshpere, therefore there is less density. so, the higher I go, the less oxygen per volume. common sense there if you have ever been in the mountains, hard to breath!
barometric pressure - storm front and other weather events have an effect on air density. the higher the pressure, the more air per volume.
Humidity - from what I can read, it has some effect, but very little compared to the other 3. basically, if there is water in the air, it takes up space, so per volume of air, there is less oxygen since some of that space is taken up by water. so, if its humid, air density goes down.
Netting all of that above out in simple up/down relationship (AD=air density):
Read the below chart like this:
*the condition = how it effects air density = the new condition it puts on your existing jetting -->which way to change your jetting to correct
*Temp hotter = AD down = richer -->lower jet number
*Temp colder = AD UP =leaner = higher jet number
*altitude higher = AD down = richer -->lower jet number
*altitude lower to sea level = AD UP = leaner-->higher jet number
*bar press Down (low pressure system) = AD Down = richer -->lower jet number
*Bar pressure up(high pressure system) = AD UP = leaner -->higher jet number
*Very Humid = AD Down = richer --> lower jet number
*very dry = AD normal = n/a -->no jet change
a good rule of thumb, for any on event in one direction, I take a guess of 2 jet sizes, or points, so I might move from a 95 main to a 98 (next hop up) for one event. if I get two of these events going in the same direction (its colder and this track is closer to sea level for example), then 4 points. if one event is going in one direction, and another event in the opposite, well they cancel each other out!
Also, for any 4 point move on the main jet in one direction, its a good idea to move your needle clip one position in that same direction.
That rule of thumb has worked for me to get it in the ballpark, but sometimes, when all else fails, revert to the tuning guide and make sure your guesses on jet directions are correct!
A common mistake for a rookie 2-stroke tuner is to confuse the terms lean/rich as it relates to air/fuel to the oil/gas ratio. it can get confusing. I used to think those two where totally different, but came across a read that shows they are related. remember how with humidity the water in the air takes up 'space' so it effects air density? well, when you put oil in gas, it also takes up space in your fuel. so for every one unit of fuel that has oil in it, there is less actual fuel! sounds picky, but seems like the professional 2 stroke mechs in the past used to fine tune air/fuel ratios track side by adjusting their 2 stroke oil/fuel ratio. if they needed to be say 1 jet size leaner, or even a half jet size, they would compensate by taking away more fuel per volume by adding more oil to the gas! tricky!
Goals of jetting
produce max power at all times. simple, yes? another way to say that is I want to keep an ideal ratio for every throttle position. for the sake of keeping it simple, let's say I just have one jet in my carb. What you need is to find out the air density and change your jet such that the ratio is 14:1 in any conditions.
Jetting howto
there are already some good guides out there that I'll link to. I don't think I can write up anything better than these.
Guide 1 (http://www.kawasakimotorcycle.org/forum/kawasaki-motocross-offroad/17227-spankeys-jetting-guide-info-2-stroke-riders-must-have.html)
More Detailed Guide (http://www.eric-gorr.com/pdfs/carb%20cleaning%20and%20jetting.pdf)
What I will add is it probably won't work for you the first time. don't quit, keep at it! its a good idea to document your jetting. that way, if you get too far off center, you can always go back to a starting place. make small moves in one direction, and usually only change one circuit at a time. I personally like to start with the pilot jet since it effects the entire range, unlike any other jet.
Here is how I document jetting:
Carb size (like 20, 21mm, etc)
main jet
pilot jet
needle position
current temp
current altitude (for different tracks)
air screw position
What motor config
Which pipe
Advanced Tools and testing
If I had a gauge on the carb the gave me real time readings on fuel to oxygen readings, it would be easy to jet. the problem is you never know that info. there are, however a few tools and tricks that can help you get your ratio right on air/fuel.
RAD gauge - or relative air density gauge. What a RAD gauge does is put all the big factors of air density into one common scale of percentage. so temp, altitude, and barometric pressure can all be netted out to tell me if I need to go up or down in jetting for any given condition. What the RAD gauge does is tell when when any of these factors go up or down enough to effect overall air density. so for example, I can be sitting trackside and see that the temp has gone up 10deg, but there is also a storm front coming in -the RAD gauge nets it all out and tells me air density has gone down 10%, and I need to go down 10% on jet size to keep my ratio correct. The down side to a RAD gauge is it won't help you jet the bike the first time. you need another method to get it spot on the first time, then going forward you can use the rad gauge to keep that ratio balanced for any given air condition.
EGT - exhaust gas temp. a neat by product of Stoichiometry is that different ratios of air:gas affect the temperature of the exhaust gas. the less fuel, the hotter, the more fuel (richer) the cooler the exhaust gasses will be. I've found this to be an effective method to get jetting dialed in for the first time. I'm not 100% sure on the ideal temp, but my best guess is that you want the exhaust to burn at 800 deg Fahrenheit under load. If I am burning at 750F, I am rich and losing power and if I am burning at 900F, I am likely about to loose my motor. for reference, in a car the ideal temp is 1200F. aluminum melts at 1200F, but the cylinders never get hot enough since the spent fuel (hot gas) is expelled so fast.
the good parts of EGT it is measurable and exact and very valuable for first time jetting or absolute checks. the down side, you need to modify your pipe to run one and also get some testing equipment to gather the probe data.