Point in the right direction for jetting please

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#1
Sorry to ask another jetting question but if I just knew which direction I should be going...

2000 Honda CR125,
DEP shorty silencer,
Handmade pipe,(aimed at top-end I think)
Wiseco piston kit,
Mainjet 45 (Stock 50)
Pilot Jet 360 (Stock)

Basically the symptoms are...
Seems to be fine above about 1/4 or 1/3 throttle but below this it accelerates VERY badly so clutching is needed. If I slow right down in 1st then open the throttle the bike bogs, almost cuts out then picks up slowly until it starts to rev then it goes perfect. Yesterday I had the bike on the stand with the throttle just off idle for about 20 minutes curing some pipe paint and by the time I was finished exhaust spooge had splashed everywhere and run all the way down the frame etc. etc. So is this rich or lean on the bottom or neither. What should I adjust? I think 45 is the smallest pilot I can put in but not sure.


thanks for any help,
 
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#2
come on, surely somebody knows should I be going lean or rich 'cause I am really confused about this whole rich/lean bottom thing...


Thanks for any help,

Forgot to say I am running Putoline MX5 at 32:1...
 
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#3
Thanks that first guide is really good and simple, I'll let you know how it goes tomorrow...

thanks
 
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#4
Ok well I think I have determined that my jet needle is lean because if I slow down then open the throttle the big kinda struggles then picks up as the RPMs increase. I moved the clip down one and it improved things greatly but I am still a bit confused about telling if it lean/rich by listening. What does pinging sound like? Is it the kind of ping you get off a bell or what? I ask because when I run the bike in about 3rd up a small hill with the throttle at 1/2 and the RPMs stabilised the bike makes that sounds kind of like an electric crackling noise, almost like missing or back firing or something. Is this pinging or what is it?


Thanks again,
 
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#5
hi there, i have the same problem on my kx 125, cause when im at the beach in about 4,5,6 gear i can hear a crackling noise, a mechanic told me that a small 2-stroke (125) will make that sound when it is under load and when the jetting is too lean, he recommended going up two sizes in the mainjet, i went from a 155 to a 157, but havent tested it yet.
 

ellandoh

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#7
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. 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.
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.
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. The reason is simple. 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 rejet 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 your jetting all across the throttle range.
Warm the bike completely, and shut it off.
As already stated, start with the pilot circuit. Turn the airscrew 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 idleing. Turn the airscrew 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 airscrew for the best response.
Now is the time to determine if you have the correct pilot installed in your carb. The airscrew position determines this for you, making it very simple. If your airscrew 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.
Once you have determined (and installed it if it's necessary to change it) the correct pilot jet size, and tuned the airscrew for the fastest idle, it's time to tune the airscrew 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 airscrew 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 airscrew is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the airscrew to compensate for changing outdoor temps and humidity. An airscrew 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.
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.
Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.
The slide is also a tuning variable for jetting, but slides are very expensive, and few bikes need different slides, so we won't go into that here.
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.
 
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#9
When you plagiarize someone elses work...you should at least give credit where credit is due ...............................
Original Thread - posted at dirtbike(dot)com in their forum on Nov 22 2003 : 6:05:13 PM

Jetting: By-Spanks

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 power band. A cleanly jetted pilot circuit can be the difference between having to clutch the bike out of a turn or not. 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.

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.

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. The reason is simple. 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 rejet 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.

Before you start the jet testing, Install a fresh plug. Warm the bike completely, and shut it off.

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 airscrew 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.

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 lean 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.

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.

Once you have a little bit of experience with jetting changes, and you start to learn the difference in feel between "rich" and "lean", you'll begin to learn, just from the sound of the exhaust and the feel of the power, not only if the bike is running rich or lean, but even which one of the carb circuits is the culprit.

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.



plagiarism

n 1: a piece of writing that has been copied from someone else and is presented as being your own work 2: the act of plagiarizing; taking someone's words or ideas as if they were your own [syn: plagiarization, plagiarisation, piracy]


Tek
 

ellandoh

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#10
this article is all over the place.... tell sparks that he should sell it it is the best ive found and will continue sharing it with anyome who can use it. unless spanks writes a better one :)
 
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#11
ellandoh said:
this article is all over the place.... tell sparks that he should sell it it is the best ive found and will continue sharing it with anyome who can use it. unless spanks writes a better one :)
I agree whole heartedly that the jetting guide that Spanks wrote is the best on the net that Ive seen so far as well.....but I also think that when someone takes their vast knowledge and experience and the time to put it into a guide to help people learn how to jet their bike properly, they should get the credit for their work...thats all I was saying.

So.....in future....maybe you could take 1/1000th the time it took spanks to organize and type it, and quote a credit to the source since all you had to do was copy and paste . :)

Tek
 

ellandoh

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#12
i was emailed this article by a friend, ages ago i had no idea where he found it . i also seen it at motocross.com, sandplanet.com, you say its at dirtbike .com and sparks has a guide i will quote spanks any time from now on its worth at least that
 
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#13
Regardless of where it came from...I sure am glad that I found this. It seems to make the most sense of anything I've found with exception to the WOT plug chop. I'm sure Rich would prefer the rider to cut the threads off and measure the carbon ring around the base of the insulator as an almost entirely white insulator can still have a more than safe a/f ratio.

The only lingering question I have (hopefully someone can offer insight) is how the choke adjustment comes into play. My 2001 RM250 has a standard setting of 12 clicks out from closed. But the manual says you can adjust anywhere from 2 to 12. So what does this adjustment do for the air/fuel ratio and at what circuit. Wonder if I should have this set at 2, 6, or 12 for starters?!

Micah
 
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#14
ellandoh
Thats great that youll give spanks credit :) I dont know him personally or anything...i just thought he should have a little glory for the great guide he put together :aj:


Micahdawg
When the choke is on it richens the mixture to get a cold engine started ...there is no point in running youre choke on while actually riding the bike (other than maybe very briefly to see if a certain carb circuit is running lean and the performance improves with a little choke) which would indicate that you need to go richer on your jetting.
Set your choke for whatever makes youre bike start well...run the engine with the choke on only until it will run without the choke on (roughly 5-10 seconds), then warm the bike up as you normally would before riding. :)

Tek