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Jetting Guide

PostPosted: 12:37 am Aug 04 2005
by Indawoods
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


PostPosted: 05:01 pm Aug 27 2006
by dirt machines
alsome thanks

PostPosted: 10:23 am Apr 26 2007
by Wazowski
Thanx for the very clear and concise explanation. I have read other descriptions like this but none quite as easily understood.

Sign of a simple mind....

Um, mine...not yours! :prayer:

PostPosted: 02:22 pm Dec 01 2009
by Normski84
Thanx for the very clear and concise explanation. I have read other descriptions like this but none quite as easily understood.

Sign of a simple mind....

Couldent agree more this was a fantastic read easy to understand, ive always been too scared to start messing with my carb just in case i cant get it back to normall this makes me want to go try it for myself now. :grin:

Cheers Inda :partyman:

PostPosted: 11:34 pm Aug 03 2011
by jonesie
Bump . Excellent thanks.

Re: Jetting Guide

PostPosted: 05:33 pm Aug 08 2014
by 3j3j3j
I should buy keihin carb jets right? Is there a way around the throttle chop test? I cant legally ride on the road and don't want people getting mad from me going 6th full throttle.can I just do it in like 3rd?

Jetting Guide

PostPosted: 05:39 pm Aug 08 2014
by scheckaet
not sure about 3rd but i did mine in 5th, maybe you can also drag the rear brake to increase engine load, just be careful...

Re: Jetting Guide

PostPosted: 05:40 pm Aug 08 2014
by 3j3j3j
Just dont wanna get people pissed

Re: Jetting Guide

PostPosted: 05:50 pm Aug 08 2014
by 3j3j3j
Is the air screw the metal gold colored one or the black plastic one

Jetting Guide

PostPosted: 08:59 pm Aug 08 2014
by scheckaet
brass one=air screw

Re: Jetting Guide

PostPosted: 01:23 am Aug 09 2014
by 3j3j3j
3j3j3j wrote:Is the air screw the metal gold colored one or the black plastic one

scheckaet wrote:plastic=idle
brass one=air screw

Thanks. I figured it quickly with a little turning of each

Re: Jetting Guide

PostPosted: 01:44 pm Sep 26 2014
by Bdub200
Thanks Indawoods! I know this is an old post but BY FAR the best jetting "how to" by a long stretch. I have spent many hours getting optimal jetting on xl600's, XS 1100, CRF230, but rarely started with the pilot. I am lucky that jetting through trial and error has had great results but probably took a lot more work without leveraging this process.

I just bought an 06 KDX200 and love it. This site has been VERY helpful with tuning and aftermarket parts. I ordered a new shock spring for my lighter weight, boyesen reeds, jets, and did lots of cleaning and adjustments (mostly suspension). So, I just donated to the cause. Anyone that hasn't should think about that value of this site and re-consider.

Re: Jetting Guide

PostPosted: 05:38 pm Nov 10 2015
by jeffersonchristopher

Brilliant, thanks for your time and effort, very informative! Cheers. :mrgreen:

Re: Jetting Guide

PostPosted: 10:03 pm Mar 27 2017
by patreilly
Indawoods, Thanks for this fantastic jetting guide. I have heard most of this at one time or another, but never in as clear and complete format. And, I can never remember all of these points. Where do I send $ to buy you lunch? I printed a copy to keep.

Re: Jetting Guide

PostPosted: 10:21 pm Mar 27 2017
by Jason
Hi. He has passed on. RIP. Please check out the link below. Thank you.