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Old 05-07-2005, 07:22 PM   #1
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Default Carb Jetting Guide

I got this guide from the z400 central website. the guy who put this up got it from Dirt Rider. I thought it might be useful. There is a picture I also have, but it doesn't seem to want to paste.

Carb Jetting Guide
You think you did everything right. Bought the most expensive pipe. Paid extra for that jet kit. Followed all the instructions to the letter. If you were lucky, the machine just might actually perform betteróexcept, of course, for that pesky off-idle stumble or the giant flat spot in the midrange, or how about the severe top end miss.
Experience tells me that almost every hop-up anybody has ever done to an ATV has produced at least one point in the rev range that carburetion is considerably worse than stock. Why? Because all you have is a piece of paper that gives you recommended jetting settings. And unless you happen to be lucky enough to ride in those same conditions, there will be some point in the rev range where you will be too rich or too lean.


Jetting is providing the engine with a combustible mixture. The ideal combustible mixture ratio is 14.7 parts of air to one part of fuel, with the most power being produced around 12-13:1. While a motor can (and will) operate on a mixture that is considerably richer or leaner, power output falls off. If you happen to go leaner and ride it hard, you may end up with an over-heated motor, or worse, a seizure.
Also be aware that carburetion is measured at throttle position settings. It has nothing to do with engine rpm or transmission gears. So telling the pipe manufacturer or Boss that it skips in third. but not fifth gear is totally useless information.
With four-strokes; the more fuel entering the engine, the cooler the piston will be. The oil and water cooling systems are not designed to cool the piston; only the little bit of fuel that is mixed with the incoming air charge prevents your motor from seizing. ONLY after the heat has been transferred through the piston to the rings and then to the cylinder, will the cooling system get the chance to do its job.


In a stock engine, the factory has spent a considerable amount of time and money trying various jets and needles to come up with jetting that not only passes the EPA (Environmental Protection Agency) regs, but allows the machine to be operated at roughly factory rated output without overheating and blowing up. When you, as an owner, change anything to do with the intake that would remove factory built-in restrictions to air flow into the engine or, exhaust changes that would do the same for air flow out of the engine, then you will need to rejet. Why?
A carburetor is designed with fixed size main and slow (pilot) jets. The jet needle attached to the bottom of the slide is fixed at a certain height. Only the idle mixture screw is adjustable. If you have increased air flow as outlined above, the increased volume will still be mixed with the same amount of fuel as before, resulting in a lean mixture. If you replace the main jet with a larger numbered jet, the jetís internal hole will be larger, thus flowing a greater quantity of fuel at 3/4-full throttle. If you raise the position of the slideís jet needle by lowering the jet needle clip, you are allowing more fuel to rise out of the needle jet at a given part throttle position which is generally 1/4-3/4 open. If you replace the low speed (pilot) jet with a larger numbered jet, the internal hole will be larger, thus flowing more fuel at very small openings of 1/16-1/4 throttle.


Even if you popped for the extra expense of a jetting kit, donít expect your jetting to be "spot on" unless you are willing to experiment and try different jets. Why? Say you install the main jet the jet kit recommends and it seems to run OK. Is it truly the best for your machine in your riding conditions? It may not be, unless you experiment by going up a jet size at a time until your machine exhibits a stumble at full throttle, indicating a too rich mixture. Then by dropping back one size you can be confident that now you have the correct jet for your machine in your riding conditions.
The same thing should also be done with the other fixed jets of your carburetor (jet needle and slow speed pilot jet.).


So, how do you start? At the bottom. Then you jump to the top and work your way down.


The idle mixture screw is the only externally adjustable carburetor jet available and controls up to 1/8 throttle only. There are two types of idle mixture screws. One type is called a fuel screw because it regulates the flow of fuel into the idle circuit. This type of screw is located ahead of the carbís slide tower (motor side) and is most often found under the carbís bore and upside-down directly ahead of the carbís float bowl. By turning the screw out you increase the amount of fuel that is allowed to slip around the tapered needle and into the carbís bore where it is mixed with air that has snuck under the carbís slide.
If the idle mixture adjustment screw is located behind the carbís slide tower (airbox side) then the adjusting needle regulates air flow into a fixed flow of fuel intended for idle. By turning this screw inward you are reducing the air flow, thus richening the idle mixture.
When the motor is up to operating temperature, set your idle speed screw to a stable idle. Then use either your idle fuel or air screw to obtain a stable idle. Reset the idle speed screw as necessary after obtaining the correct idle mixture.


The main jet controls 3/4-full throttle only. Ideally you should start very rich (large numbered jet) and test at full throttle. It should skip. If not then you are not rich enough! Once you have your rich stumble, back off one size at a time until full throttle operation results in normal operation. (Note: If your ATV runs faster at 3/4 throttle than full throttle you are definitely lean on the main!)


The slideís jet needle controls 1/4-3/4 throttle. It does this by passing upward through the needle jet. The needle jet is a long brass tube that contains many small holes in its sides that air passes through. Fuel from the float bowl enters this air stream from the main jet and into the center of the needle jet where it mixes with the air to create an emulsion. This mixture of fuel and air is then metered by the height, taper and diameter of the jet needle as the emulsion passes upward around the jet needle into the carbís bore where it mixes with still more air to (hopefully) arrive in the motor in a combustible fuel-to-air ratio.
If you have a soft hesitation, without a hard stumble, anywhere between 1/4 and 3/4 throttle, chances are your needle is lean, so raise the needle by lowering the clip. Conversely, if you have a hard stumble, chances are the needle position is rich, so lower the needle by raising the clip.
If you get very unlucky you might have to start playing with jet needle taper which controls how fast the mixture increases as the jet needle is raised. This would come into play if you were lean at 1/4 throttle, yet rich at 3/4 throttle. The length of the needle comes into play here too. The diameter of the needle controls how much fuel escapes around the needle while still inside the needle jet. The larger the diameter of the straight section or "L" length, the leaner the mixture. Or finally, the "L" length, which controls how much the slide rises before the tapered part of the needle starts.

SLIDE CUT-A-WAY(I don't think anybody adjusts this one)

The slide cut-a-way controls the amount of air allowed to pass under the slide at 1/8-1/4 throttle. It controls the transition from the low speed (pilot) jet to the main jet-fed needle jet/jet needle. Replacing the slide with one that has a smaller number (less cut-a-way) will decrease the amount of airflow under the slide at 1/8-1/4 throttle openings, thus creating a richer mixture at that throttle opening. If you have a rich condition at 1/8-1/4 throttle and you canít go any leaner, try a smaller cut-a way.
But thankfully, jet needle taper, diameter, "L" length and slide cut-a-way are usually not affected by most simple pipe/air filter modifications.


The low speed (pilot) jet controls fuel flow at 1/8-1/4 throttle. The low speed (pilot) jet is usually not affected by most simple pipe/air filter modifications. However, a slightly lean low speed (pilot) jet can raise havoc in the winter where its fuel is added to the total mixture strength required to start. You may find going one level up will help a winter cold start situation.
Finally your idle mixture is revisited if you have a deceleration backfire situation. When you chop the throttle and use the motor to decelerate, if you get a stream of backfires, try increasing your idle mixture strength 1/4 turn at a time until the backfire goes away. Note: If you reach a point where your idle mixture is 4 turns out (for fuel type screws, NOT air type screws), try going up one size on the slow speed (pilot) jet and reset your idle mixture screw to 1-1/2 turns out and repeat the process.

One final note; reading about how to jet will not make you "good" at jetting. And asking someone a thousand miles away why your machine skips in third gear wonít get you the answers you seek. Even Boss canít help you when you write him asking "I just bought an XYZ pipe. What jet do I need?"Only hands-on, trial and error experience can solve your jetting problem.

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Old 05-07-2005, 10:57 PM   #2
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Default Carb Jetting Guide

This is for a Warrior Carborator

This was done up by my bud Warrior03 on bluetraxx...very helpful....if you have any problems ask in this thread and we will do our best to help you 8-)

This is what you are looking at when you disconect the air boot clamp and the intake manifold bolts.

This is what the carb looks like with the side cover off..... you don't need this to rejet but you can use this to check the accelerator pump in the bottom right hand corner of the carb.

Now, looking at the top of the carb you can see the cover on.

If you take that cover off, you will see the lift arm, and slide. In order to ge the slide out, you have to remove the 2 little phillips head screws down on that white plastic piece that is connected to the lift arm.

Once you do that, you will have the slide out and in your hand.

This is a picture of the slide with the plastic piece with the screws installed.

After you remove that white plastic piece.... you will have to push up on the needle so it comes out of the slide, grab it, pull it out and it will look like this.

All you have to do is simply take the little e clip off of the needle and move it to a different groove. The higher you go the leaner it makes it. The stock e clip postition is the 3rd or middle groove. After you change the position, just reassemble everything in the reverse order.

Next, is the main and pilot jets. The stock main jet is a 145 and the stock pilot is a 42.5.

This is a picture of what the float bowl looks like not removed.

And this is what it looks like removed. The big brass "screw looking thing" in the center is the main jet. The brass "screw looking thing" down in the hole is the pilot jet. Change these if necesary and reassemble in reverse order.

While you have your float bowl off, you might want to take the spring that you can see on the left of the carb and stretch it. This is a failure waiting to happen to alot of warrior owners. The spring becomes worn out and does not work properly. So just stretch it out and reinstall it.

Next is the air/fuel mixture screw. Adjust to your specific needs. Out is richer and in is leaner. here are two pics of it.

You can see it here - it is the screw above the hole in the bottom of the carb in this pic. The stock setting is 2 turns out for your referance. Try adjusting this a half a turn at a time to perfect your jetting.

And here is a pic of it with the float bowl on. It is still adjustable!

After all of these steps are done, reinstall everything in reverse order.

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Old 08-29-2005, 04:26 PM   #3
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Default Carb Jetting Guide

Very VERY helpful! My baby is running like a scalded cat thanks to this guide. I've never jetted before, and I was screwing around with the needle. It pulled like a banshee until full throttle and got a bad stumble there, and when I finally changed the main jet it pulls all the way (I put it on top of me yesterday going up a ditch!). For those who don't have the Edlebrock yet, mine is a Mikuni VM 36mm round slide with a 6FJ11 needle and a 180 main jet (cheap assed cobra pipe for now). White bros cam, etc, etc.

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Old 01-31-2006, 11:51 AM   #4
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Default Carb Jetting Guide

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Old 05-16-2006, 10:42 AM   #5
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Default Carb Jetting Guide

how do you do this on an edelbrock...i like theese instructions....im a visual learner
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Old 05-16-2006, 01:30 PM   #6
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Default Carb Jetting Guide

how do you do this on an edelbrock...i like theese instructions....im a visual learner
Then stop living your life with your eyes closed!!!
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Old 05-17-2006, 07:15 PM   #7
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Default Carb Jetting Guide

If anyone has this but on a raptor carb let me know... after i get my parts and it still bogs now in the middle of 1st i think i might try this...
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Old 09-08-2006, 11:03 PM   #8
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Default Carb Jetting Guide

hey i just got a 02 warrior 350 and when i give it full throtle it dies or back fires do u think by fallowing this steps it would fix the problom ?
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Old 09-27-2006, 01:41 PM   #9
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Default Carb Jetting Guide

I tried going by this and no help, so i tried by DynoJet's Guide and still no help.

It idle's perfect but when running it up to around 3000 RPM it start's missing badly, i can do a quick ramm and release on the throttle and it sounds perfect, but yet once i hang it at a certain RPM it misses again.

Even when i cram the gas down it start's reving real quick but at a certain peak it try's to die (yeah i know that's the thing with DynoJetKit's) but missing at a certain RPM even at 3k it shouldnt do it.

With stock Jetting it never missed.

So could it actually be missing or does it just sound like it's missing because of more fuel flow?

BTW, i know it isnt the Cobra Exhaust, it was sounding fine with stock jetting.
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Old 09-29-2006, 12:39 PM   #10
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Default Carb Jetting Guide

The problem would be the needle is on the 4th groove (recommended by DynoJet with the 148 Jet). However i will go back to the 3rd goove and see if that help's.

Racing this weekend and i want it to run right, but while it's in gear and going it run's fine, just the sitting still reving is when it start's missing.
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Old 02-22-2007, 09:35 PM   #11
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Default Carb Jetting Guide

i got an fmf pipe and mid pipe but stock header and i removeed the snorkal on the air box....to get the best performance is their n e rejetings that i should do?
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Old 02-27-2007, 04:36 PM   #12
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Default Carb Jetting Guide

I'm having a problem. How do you get the second phillips head screw from the white plastic? It's under the lift arm and I can't get a screwdriver to fit in there.. Please help!
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Old 02-27-2007, 06:14 PM   #13
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Default Carb Jetting Guide

I'm having a problem. How do you get the second phillips head screw from the white plastic? It's under the lift arm and I can't get a screwdriver to fit in there.. Please help!
It's tricky to get to. Usually I took out the screw that holds the lifter arm onto the shaft first, to make it less of a struggle to work with. Then push the slide up from underneath and hold it there with one hand while you unscrew it with the other. That's the best way I found to do it (assuming I am remembering this correctly, I haven't messed with a stock carb for a while)

-2008 Raptor 700
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Old 12-11-2009, 11:14 AM   #14
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Default Carb Jetting Guide

I know i need to rejet but i dont know if i need to rejet the main and pilot. i just have a k/n with an open aribox (holes). The problem im having is a quick jab and sheboggs spits and eventually dies if you hold the throttle. a steady increase and let go and it sounds good. and at full throttle in any gear, it boggs. just want to confirm what i need before i go back to the dealer again.
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Old 12-11-2009, 06:45 PM   #15
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Default Carb Jetting Guide

same here!^
K&N air-filter, Supertrapp pipe, Dynojet stage 2 jet kit, Raptor 660 rear shock, 5" front wheel spacers, Race cut front fenders,Tusk comp series nerf bars, front bumper, parking brake block off plate, Tusk aluminum gas cap, Metallic black frame, Blue a-arms/swing-arm, MSR handle bars, MSR bar pad & clutch perch, PRM Rear swing arm skid, Custom light set up, YZ 250 shifter, Drilled/ slotted rear brake disc! And more!!

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