gassrx

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 mid-range, 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 re-jet.

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. Only hands-on, trial and error experience can solve your jetting problem. So go purchase a handful of jets and get your hands dirty!




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      Part numbers:
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      Now, the tools you will need are as follows:

      ~A collecting cup of some sort. I used a peanut butter jar.
      ~Ratchets for the following sizes:
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      - Extension for the sockets needed
      ~Phillips and Flathead screwdriver (Be sure these are in perfect condition. A badly worn screwdriver will strip the screws)
      ~Needle nose pliers
      ~”Vise grips” or known as locking pliers (Two)
      ~Open end wrench 7mm and 12mm
      ~ It’s a good idea to have a extra hand around
      (Not needed, but I highly recommend tiny Phillips and flathead screwdrivers (Pictured next to the jar and the ¼” extension) I recommend these for removing a couple things since you can put pressure with your thumb on the end and unscrew it with the other hand. This insures that you will not over tighten any parts, and ensure that you will not strip the heads of the bolts.
      Ok, now that you have the tools, let’s start by putting the bike on a bike stand. I put it on the stand rather than the kickstand because it’s more stable and sits higher. I hate working on my knees. Start by taking the number plates off. Yes, both of them. The right side, you take off one bolt and the top comes off of its rubber grommets, pull the top off, and the plate comes right off. The left hand side, use the 10mm socket to take the battery bolts off, and then take the Phillips bolt near the back. Again, rubber grommets are used to hold the top in place. Take the seat off. There are two mounting bolts on the back:

      Those two bolts are both a 12mm socket. Use the open end wrench on the inside, and use the socket on the outside. You may need to use an extension if you don’t have a deep socket. Once you have the two bolts off, slide the seat back, and lift it up. This is what you have. Notice there is a hook in the middle and a knob on the tank. That is what you are sliding the seat off of.

      Now that the seat is off, you must take the gas tank off. Don’t worry, you won’t spill any gas any where, I promise. On the left hand side of the bike where the valve is, slide down the metal clip holding the tube in place. Turn off the gas supply, and slip the tube off slowly. Now take off the two bolts in the front of the take. This is on the lowest part of the gas tank in the front, behind the tank shrouds. The socket you will use is an 8mm socket. Take the bolts all the way off and set them aside. Now look back at the last picture posted. On the back of the tank, there is a rubber piece connected to the knob and the frame. Slip that rubber piece off of the frame. Pull the vent tube out of the steering stem and lift the tank up. Don’t tip it, and lay the tank aside where you won’t trip on it. This is what you’ll end up with:

      It may be a good idea to take a rag, and wipe all the dirt off the top of the bike if any. You don’t want anything dropping down into the carb. If you do, engine damage is the result. A clean bike is always a good thing! Now we must drain the gas out into that container. This is very easy. Make sure you open the garage door, windows, whatever, to let the fumes out. Breathing this crap is bad. Here is where the drain screw is:

      (Don’t worry about removing the carb, that comes later) This is on the right side of the carb, on the float bowl. The vent tube that goes down to the bottom of the bike is where the gas drains to. Put the jar under that tube and start to unscrew that screw, enough so that the gas leaks into that jar. Once the gas doesn’t drip anymore, close the screw all the way. Now on to the top of the carb. We are going to take this cover off:

      This cover comes off by removing the two screws. Once removed, the lid comes off as well as the gasket. Flip it over and set it aside. Do not set the gasket side down on the ground, as it will get contaminants! Here is what you are facing:

      The angle of the camera cannot show the two screws. But one is visible. It has a red dot, and opposite of that side is a darker red dot. I made it darker because it’s not visible, but that is where it is. This is where I use the miniature screw drivers to get the screws. I magnetize the screwdrivers, and use care to make sure I don’t strip the heads. Metal pieces in a piston are not good! Remove the two screws. Put these screws on a clean surface so they do not get contaminants. Now get your vise grips and set it so that it will lock onto the throttle, not too tight, not too loose. Set the vise grips on the seat. Start to open the throttle slowly as you guide that “plunger holder” (as I call it) up to the top. Once you have the throttle all the way open, take the vise grips, and lock it so that the throttle does not go back any more. What I do is I hold it pinned and lock it up against the brake so it doesn’t rewind on me. If you don’t have locking grips, a friend will do, just have them hold the throttle open all the way until you are finished. How fold the plunger holder to the back of the carb and pull the piece up to the top. Take care not to remove it, as it is a pain to get back together! If it came apart on you, this is what it should be assembled to:

      Once you get the holder out of the slider, set it back like this:

      As you can see, the bar is back 45 degrees, while the holder is forward 45 degrees to make a S. Here is what you are faced with when you look down on the carb:

      Where the red dot is where the needle lies. Grab needle nose pliers and carefully pull up the needle out of its slot. This is what the needle looks like once it is out.

      Now we must move the carb to take the bowl off. Untie the two straps on the front and back of the carb. Don’t take them off; just loosen them until the threads are at the end. Take the front of the carb off the boot and twist the bowl as much as you can towards you. Tie the back tie down to that it does not rewind back on you. This is what you have:

      Now we must take off the bowl. Some people take that hex nut off to change the main jet, which you can, but you cannot access the pilot jet, and you can’t take out the needle jet (a piece the needle slides into), so we need to take it off. It’s just three bolts. As we look at the underside of the carb, this is what you will see:

      The bolts with the red square dots are the bolts you will be removing. These are Phillips head bolts, and the bolt with the blue dot is your fuel screw. This is what you will adjust when the time comes, but keep in mind where that bolt is. You need a small flat blade to adjust it.
      Well, take those screws off, and you are faced with this:

      The blue dot is for cross reference, which is the fuel screw once again. The green dot is the pilot jet. You can remove this using a flat blade screwdriver. Just unscrew it and pull it out. Once you pull it out, set it aside and put in the 45 pilot jet you got. The red dot is the main. You remove this by using a 6mm socket. Just unscrew it. If the whole thing turns, not just the jet, but the 7mm sized socket under it, don’t worry, that piece has to come out as well. If it doesn’t, use a 7mm to unscrew it off. Here is what the jets look like:

      Pilot Jet

      Main jet attached to the tube. Take the main jet off by using an open end wrench and a socket on the jet. Again, it screws right off.
      Here is what you are faced with if you look form the bottom up.

      From left to right: Main jet, Pilot Jet, Fuel screw. Now in the main jet’s hole, if you look closely, you see a bronze piece in the middle of that hole. We are going to take this off. Since I did not do this part (I only changed my pilot jet when I took these pictures) there are no pictures taken for this section but this is really simple to do if you’ve been a good student and know where things go. You should know anyways, you have to put the bike back together!
      (Notice: There have been discussions about these needle jets being the same. Only change this needle jet if the one you have is worn out. If you do not have the old needle, a older drill bit bigger than 3/20ths (.150), and smaller than 11/100 (.11") Use the tapered side of the bit, set it down in the hole and tap it out carefully.)
      Now take your OLD needle, I repeat, the OLD needle because what you are going to do next will ruin it. Pull the clip off with your needle nose pliers, or a tiny screwdriver to pry it off. Then put the needle back in the hole where it goes. That’s right, just to clarify, you took off the needle, and you put the needle back in the hole with no clip. Slide the point side first, just as it would go normally. Now if you look at the bottom of the carb, the needle is protruding past the main jets hole. Grab another pair of locking pliers (vise grips as I call them) and lock it as tight as you can on the needle. Pull with all your might on the needle. Use two hands. Have a friend hold the carb so you don’t pull it off the boot. Tell them to stick their fingers in the hole that goes to the engine, and pull up. After pulling hard, the needle jet should slip right off. Then notice which side goes towards the top of the carb. There is one side that is a smaller diameter than the other. Take the new needle jet, and push it up into the hole the way the old one was set. Just get it straight. Take the tube the main jet goes into, and start threading it in. Once you can’t tie it down anymore with a ratchet, unscrew it and look at the needle jet to make sure it’s set. That’s it for the needle jet. Now let’s start putting the carb back together.
      (Notice: Many people have destroyed jets and such by overtighting them! Use the thumb on the head of the wrench and two fingers on the wrench to tighten it down.)
      Thread the main jet into the tube it goes into, and then start putting it back on the carb. Thread the pilot jet in as well if you haven’t done so already. Remember these carburetor metals are soft as cheese, so don’t over tighten the jets very much. What I do is I put my thumb on the top of my ratchet, and use two fingers closest to the head of the ratchet to tighten the jet. That’s how tight I go when I tie them back in.
      Now before we put the carb back together, let’s adjust the fuel screw. Take a small screwdriver, and start screwing in the fuel screw until it sets. Again, do not over tighten, just let it set. Then count back your turns. Count back 1.75 turns.
      Now we must put the bowl back on. The white piece that came off with the bowl goes back as followed:

      If you look directly under the carb, the round hole is aligned with the pilot jet. Take the float bowl, and put it back on.
      Untie the rear clamp and the front clamp as well. Slip the carb back the way it used to. Make sure that it is straight up and down with the rest of the bike. The notch on the front boot should be aligned with the notch on the carburetor, and the notch on the carburetor should be in that slot. Tie the clamps down securely.
      Let’s put the needle in. These are how the needle numbers go:

      The top clip position is #1, the lowest one, closest to the bottom, is #5. (The picture says six but it is five in this case) For reference #1 is the leanest position, while 5 is the richest. I put the clip in the 4th position. Read at the bottom of the page and you can know what conditions I ride in, and you can adjust them to your preference.
      Put the clip in the new needle, slip it in. Take the vise grips off your grips and start guiding the plunger holder down to the bottom. Remember not to let that assembly come apart because it is a pain in the ass to get it back together! Once you get it to the bottom, put the two screws on, and then put the cover on.
      Now that you have done the carburetor mods, there is still one thing you want to do to complete the process. Don’t worry, this takes less than a minute! On the top of the air box there is a snorkel:

      As you can see, you can slip your fingers in and pull it out. Do that. This lets more air in to the air box. Don’t worry about water getting in. There is a lip that is about 1/8” high that doesn’t let water in. When you wash, don’t spray a lot under the seat, but don’t worry about it too much.
      The next thing you must do is remove the exhaust baffle. The screw is a torx type, or you can carefully use an allen wrench and take care not to strip it:

      The screw is at the 5 o’clock position and all you do is unscrew it, reach in, and yank it out. This setup still passes the dB test. The bike runs 92 dB per AMA standards, which is acceptable. Just carry this baffle in your gear bag if the ranger is a jerk off. I’ve never had a problem, but don’t take chances.
      That’s it! Start putting your tank on, seat, and covers. After you put the seat on, pull up on the front, and the middle of the seat to make sure the hooks set in place.
      Turn on the bike, and take a can of WD-40. Spray the WD-40 around the boot where it meets the carburetor. If the RPM rises, you know you have a leak, and the leak must be stopped. You must do this to make sure there are no leaks!
      Here is my configuration:
      04’ 230F
      Uni Air filter
      132 Main Jet
      45 Pilot Jet
      Power up needle, 4th clip position
      Fuel screw 1.75 turns out
      Riding elevation: 2000ft - Sea level
      Temperature – Around 60-90 degrees
      Spark Plug Tips
      When you jet your carb, a spark plug is a best friend. Make sure your spark plug is gapped correctly, (.035) but that’s not all that matters. You want to make sure the electrode is over the center, and you want the electrode to be parallel, not like a wave of a sea. Put in the plug, and run the bike for 15 mins, ride it around too then turn it off. Then take off the spark plug after letting the bike cool. The ceramic insulator should be tan, like a paper bag. If it is black, it is running rich, if it is white, it is running lean. The fuel screw should be turned out if it is running lean, and turned in if it is running rich. Go ¼ turns at a time until your plug is a nice tan color.
      Making sure your bike is jetted correctly
      While you are running the bike for those 15 mins to check the plug color, you want to make sure it’s jetted correctly now. Here is what the jets/needle/screw control:
      0- 3/8 throttle – Pilot jet
      ¼ to ¾ throttle – Needle
      5/8 – full throttle – Main jet
      0-Full – Fuel screw
      Pin the gas, does it bog much? Just put around, is it responsive? When you’re coming down a hill, the rpm’s are high and you have no hand on the throttle, does it pop? If it pops, it is lean and the pilot jet should be bigger. If it’s responsive your needle is set perfectly. You shouldn’t have to go any leaner than the 3rd position, but I put mine in the 4th position to get the most response. Your bike shouldn’t bog much when you have it pinned. If it does it is too rich of a main jet.
      Determining the plug color, you will have to mess with the fuel screw.
      That’s it, have fun jetting, and any questions, post on the forum, but remember to do a search first.
      Also, if your bike requires different jets due to alititude, humidity, or temperature, please post the following so we can better assist you:
      Average temperature
      Altitude (If you do not know this, there is a link in the Jetting forum that you can look up your alititude)
      Average Humidity
      What jets you are currently running
      What the problem is (If there is one)
      Just do that and we'll help you out the best we can.
      EDIT: The girl using this login name is my girlfriend. You can reach me on my new login name at 250Thumpher
      Then again, you're more than welcome to say hi to her!
      -Phill Vieira