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***Carb Jetting Guide***

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 better—except, of course, for that pesky off-idle stumble or the giant flat spot in the midrange, or how about the severe top end miss (must be the darn rev limiter!).
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.

WHAT IS JETTING

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.
Did you know that your fuel pre-mix ratio (two-strokes only!) affects your jetting? A carburetor jet flows X amount of fuel and air at a given time. In that fuel is, say, 32 parts of fuel and 1 part of oil (32:1). If you change your pre-mix ratio to 20 to 1 because you are afraid of burning up your motor, all of a sudden the amount of fuel has been decreased by 37.5 percent. And since it is the fuel and NOT the oil that keeps your ring-ding cool, you run even leaner and hotter! Same theory applies to four-strokes as well; 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.

WHY DO I HAVE TO REJET?

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 jet’s internal hole will be larger, thus flowing a greater quantity of fuel at 3/4-full throttle. If you raise the position of the slide’s 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.

I STILL FEEL I CAN JET BETTER THAN THE FACTORY CAN

Even if you popped for the extra expense of a jetting kit, don’t 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.).

WORKING WITH INDIVIDUAL CARBURETOR CIRCUITS

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

IDLE MIXTURE SCREW:

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 carb’s slide tower (motor side) and is most often found under the carb’s bore and upside-down directly ahead of the carb’s 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 carb’s bore where it is mixed with air that has snuck under the carb’s slide.
If the idle mixture adjustment screw is located behind the carb’s 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.

MAIN JET

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!)

JET NEEDLE

The slide’s 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 carb’s 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

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 can’t 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.

LOW SPEED (PILOT) JET

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 won’t get you the answers you seek. Even Boss can’t 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. So go purchase a handful of jets and get your hands dirty! You are out five big ones for that pipe and jet kit, and now have a hobbling pile. What are YOU going to do about it?



Enjoy - this article was from the Dirt Wheels Tech Tips... Hope this helps!
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dang, i think that's the most useful, knowledgable post i've ever seen on this board, thanx SpaGuru, that really helped
 

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Discussion Starter #3

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Good info, but still need more!

Anyone,

I'm about to remove my airbox cover, and possibly run an aftermarket filter and outerwears cover. I'm pretty sure I can do the jetting myself, but does anyone have any detailed pictures of the inside of the carb? I'm thinking about buying a service manual, but not sure if the carb is covered in there. I'm pretty technical, but have heard horror stories about stripped bolts, etc. I'd like to do it right the first time. Any on-line pics and/or tips would help me a lot.

Thanks,
 

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The only problem with anything stripping are the screws that hold the bowl onto the carberator. There are 4 small screws that hold the bowl on. Some people have stripped the phillips slots, but you can still get them out. Replacing the main jet is real easy.

The hardest part is figuring out which pieces of plastic to remove. I removed the three pieces that cover the gas tank. I loosened the two front fenders though to make it easier to remove gas tank cover plastics. I then removed the gas tank. Remember to pinch off the larger fuel line coming from the tank or you'll have gas all over. I removed the other end of this line from the carb. There are only two screws holding the gas tank in. There is a smaller hose comming from the tank to the carb. I think it's a vacuum line. Unplug this from the carb also. Now you can pull the gas tank free of the quad.

Next there are two clamps that hold the carb into the intake tube. Once you've loosened these clamps, you can pull the carb up through the top of the quad. At this point it's still connected to the throttle cable. I would leave this attached. You can turn the carb over to get to the screws.

From here you can remove the four screws from the bottom of the bowl. Oh ya, at this point if there was any gas in the carb, it has leaked all over.

Now that you've removed the bowl, you should be able to see the main jet. It sits in a bowl. Take a slotted screw driver and remove. You may have to hold the hex nut underneath the bowl with an open end wrench to turn the main jet. Once jet is removed, place new jet in.

Now to get everything back together again without having any extra pieces left over!

Hope this helps.
 
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Thanks for the info. Doesn't sound too bad. I was looking around Mikuni's web site, and they've got tons for documentation for the HSR and RS series. Nothing on our BSRs though. Would these be close to what they have? Also, where are the needle valve and pilot jet in relation to the main jet?

Thanks again,
 

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where to get a service manual

Where can I get a service manual for my z? The dealer or elsewhere?
Thanks
Steve
 

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Go to Kawasaki's web site, and you can order it from there. Got mine in about 3 days.
 

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I removed the seat, and front plastic but left the gas tank connected and hanging off of the side. Then the little black plastic crankcase breather was unbolted and the two top hoses removed and put on the same side of the gas tank to make rooom to turn the carb. I loosened the two clamps that held the carb and twisted the carb until I had it sideways enough to use a screwdriver for the bowl screws, then I disconnected a few hoses that were in my way which were mainly breather hoses. I removed one bowl screw and stripped the other three. The stripped screws were removed with a drill, after you take the heads off the screws the rest are easily removed with your fingers. The bowl screws were replaced with 4x10mm stainless allen head cap screws.

In all, it took 3 hours to install a full Yoshi comp system, a dynojet kit and a pro-flow filter setup. I unbolted the radiator's "three" bolts and had enough room to remove the stock header pipe without spilling a drop of coolant or cutting the pipe in half. I tried to remove the header pipe without removing anything else but I gave up...
 

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Does the pilot jet have a long aluminum splashguard around it?
 

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Re: Good info, but still need more!

QUOTE
Anyone,

      I'm about to remove my airbox cover, and possibly run an aftermarket filter and outerwears cover.  I'm pretty sure I can do the jetting myself, but does anyone have any detailed pictures of the inside of the carb?  I'm thinking about buying a service manual, but not sure if the carb is covered in there.  I'm pretty technical, but have heard horror stories about stripped bolts, etc.  I'd like to do it right the first time.  Any on-line pics and/or tips would help me a lot.

Thanks,[/b]
its pretty easy all u have to do is remove the gas tank and some plastic take the carb out get an impact wrench(make sure u hit it hard or it will strip out) take the float bowl screws out and its right there take it out with a flat head screwdriver incase u dont know it the one insie the splash bowl
 

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if you want to do work on your carb like the pro's, then you need a tool the pro's use. this tool will set you back a whole $10.00 (on average ). what's the tool you say? it's a impact screwdriver! i have yet to strip a carb screw using this tool. money well spent! you can pick one up at any auto parts store.
 

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We need one of these for the FCR40.
 

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Thanks for posting that info, that was exactly my problem.
 

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Hey all that I do is just take off the main fuel line, and get a pair of needle nose vescripts and pinch the line since the z's dont have an "off" position. I had alot of problems trying to get the screws out that held the bowl on. Finally I had to take out the whole carb and use viscrips to get them out. I had to get new screws. But if you don't have a prob. with getting them out, then first off, just loosen the 2 air clamps from your air box and the one that goes to your motor. Once you do that you can twist the carb towards yourself just enough to pull the bowl off. Then you can go from there. The main jet is hard to miss. It has a splash guard around it and I think you'll need like a 1/4" wrench to loosen it. (Make sure not to lose the O-ring on the bowl, and the one on the splash guard) Once you get the guard out then loosen the part that goes down into the carb from the guard and the jet should fall right out. Sorry if this isnt very detailed.
 

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Thank you for posting this diagram. I don't even want to get into what happened to me. Let's just say this cleared up alot of things for me. Thanks again.
 

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Right now my quad motor is stock. I have a k&n on the way. But anyway. Today i went riding and the quad seemed to run weird. It would almost run like it was running out of gas. Full throttle it would kind hessitate. And from reading this "If it runs better 3/4 throttle then full then your lean on the main" Well i know im lean then so this thread is helpful. But anyway if im lean on the main stock then what should i do when i get the K&N?
 

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Don't make youself too much work changing mains. Pop the seat, loosen the snorkel clamp just in front of the air box and pull the snorkel out (pull the top carb breather off the carb with the snorkel). Loosen the front and rear carb clamps. Put the bottom carb drain hose in a pop bottle and open the drain screw. This will eliminate any spilled gas. Pull the rear carb boot towards the airbox and pull the carb out of the front boot. A little pull down and twist towards you and the carb is out. You can pull the parking brake cable pin clamp from the frame that gets in the way of pulling the back of the carb out towards you. No need to pull plastic or anything else.
 

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just jetted my carb today

I thank you personally for making this awesome thread and taking the picture of the carb!

i followed all directions from dyno jet and have a 150 main jet with E clip in the 3rd position.

also installed my airbox outerwear (kinda tricky)
and a curtis sparks x-6 full exhaust ( stock header was a w00ter to remove that is definitely where majority of time was spent...

all in all quad runs like a champ so much more low-end then before havent yet to open it up as it was very dark when i finally completed the task.

THANKS AGAIN!!
 
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