the problems of jetting
this is great worth reading

Before changing jetting you should first check for any intake leaks.  Carb cleaner works great but water will do.
  Start your quad or bike . Hold throttle at stead  idol spay the intake if there is a spike in the RPM's, you will then need to fix the leak. Don't over torque the intake bolts it will warp, making the leak worse.

make one change at a time .
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. fast or last

Motorcycle Carburetor Theory 101

Motorcycle carburetors look very complex, but with a little theory, you can tune your bike for maximum performance. All carburetors work under the basic principle of atmospheric pressure. Atmospheric pressure is a powerful force which exerts pressure on everything. It varies slightly but is generally considered to be 15 pounds per square inch (PSI). This means that atmospheric pressure is pressing on everything at 15 PSI. By varying the atmospheric pressure inside the engine and carburetor, we can change the pressure and make fuel and air flow.

Atmospheric pressure will force high pressure to low pressure. As the piston on a two stroke engine goes up (or goes down on a four stroke engine), a low pressure is formed inside the crankcase (above the piston on a four stroke). This low pressure also causes a low pressure inside the carburetor. Since the pressure is higher outside the engine and carburetor, air will rush inside the carburetor and engine until the pressure is equalized. The moving air going through the carburetor will pick up fuel and mix with the air.

Inside a carburetor is a venturi, fig 1. The venturi is a restriction inside the carburetor that forces air to speed up to get through. A river that suddenly narrows can be used to illustrate what happens inside a carb. The water in the river speeds up as it gets near the narrowed shores and will get faster if the river narrows even more. The same thing happens inside the carburetor. The air that is speeding up will cause atmospheric pressure to drop inside the carburetor. The faster the air moves, the lower the pressure inside the carburetor.

Most motorcycle carburetor circuits are governed by throttle position and not by engine speed.There are five main metering systems inside most motorcycle carburetors. These metering circuits overlap each other and they are:
* pilot circuit
* throttle valve
* needle jet and jet needle
* main jet
* choke circuit

The pilot circuit has two adjustable parts, fig 2. The pilot air screw and pilot jet. The air screw can be located either near the back side of the carburetor or near the front of the carburetor. If the screw is located near the back, it regulates how much air enters the circuit. If the screw is turned in, it reduces the amount of air and richens the mixture. If it is turned out, it opens the passage more and allows more air into the circuit which results in a lean mixture. If the screw is located near the front, it regulated fuel. The mixture will be leaner if it is screwed in and richer if screwed out. If the air screw has to be turned more than 2 turns out for best idling, the next smaller size pilot jet will be needed.

The pilot jet is the part which supplies most of the fuel at low throttle openings. It has a small hole in it which restricts fuel flow though it. Both the pilot air screw and pilot jet affects carburetion from idle to around 1/4 throttle.

The slide valve affects carburetion between 1/8 thru 1/2 throttle. It especially affects it between 1/8 and 1/4 and has a lesser affect up to 1/2. The slides come in various sizes and the size is determined by how much is cutaway from the backside of it, fig 3. The larger the cutaway, the leaner the mixture (since more air is allowed through it) and the smaller the cutaway, the richer the mixture will be. Throttle valves have numbers on them that explains how much the cutaway is. If there is a 3 stamped into the slide, it has a 3.0mm cutaway, while a 1 will have a 1.0mm cutaway (which will be richer than a 3).

The jet needle and needle jet affects carburetion from 1/4 thru 3/4 throttle. The jet needle is a long tapered rod that controls how much fuel can be drawn into the carburetor venturi. The thinner the taper, the richer the mixture. The thicker the taper, the leaner the mixture since the thicker taper will not allow as much fuel into the venturi as a leaner one. The tapers are designed very precisely to give different mixtures at different throttle openings. Jet needles have grooves cut into the top. A clip goes into one of these grooves and holds it from falling or moving from the slide. The clip position can be changed to make an engine run richer or leaner, fig 4. If the engine needs to run leaner, the clip would be moved higher. This will drop the needle farther down into the needle jet and cause less fuel to flow past it. If the clip is lowered, the jet needle is raised and the mixture will be richer.

The needle jet is where the jet needle slides into. Depending on the inside diameter of the needle jet, it will affect the jet needle. The needle jet and jet needle work together to control the fuel flow between the 1/8 thru 3/4 range. Most of the tuning for this range is done to the jet needle, and not the needle jet.

The main jet controls fuel flow from 3/4 thru full throttle, fig 5. Once the throttle is opened far enough, the jet needle is pulled high enough out of the needle jet and the size of the hole in the main jet begins to regulate fuel flow. Main jets have different size holes in them and the bigger the hole, the more fuel that will flow (and the richer the mixture). The higher the number on the mainjet, the more fuel that can flow through it and the richer the mixture.

The choke system is used to start cold engines. Since the fuel in a cold engine is sticking to the cylinder walls due to condensation, the mixture is too lean for the engine to start. The choke system will add fuel to the engine to compensate for the fuel that is stuck to the cylinder walls. Once the engine is warmed up, condensation is not a problem, and the choke is not needed.

The air/fuel mixture must be changes to meet the demands of the needs of the engine. The ideal air/fuel ratio is 14.7 grams of air to 1 gram of fuel. This ideal ratio is only achieved for a very short period while the engine is running. Due to the incomplete vaporization of fuel at slow speeds or the additional fuel required at high speeds, the actual operational air/fuel ratio is usually richer. Figure 6 shows the actual air/fuel ratio for any given throttle opening.

Carburetor troubleshooting is simple once the basic principles are known. The first step is to find where the engine is running poorly, fig 7. It must be remembered that carburetor jetting is determined by the throttle position, not engine speed. If the engine is having troubles at low rpm (idle to 1/4 throttle), the pilot system or slide valve is the likely problem. If the engine has problems between 1/4 and 3/4 throttle, the jet needle and needle jet (most likely the jet needle) is likely the problem. If the engine is running poorly at 3/4 to full throttle, the main jet is the likely problem.

While jetting carburetors, place a piece of tape on the throttle housing. Place another piece of tape on the throttle grip and draw a line (while the throttle is at idle) straight across from one piece of tape to the other. When these two lines are lined up, the engine will be idling. Now open the throttle to full throttle and draw another line directly across from it on the throttle housing. At this point, there should be two lines on the throttle housing, and one on the throttle grip. Now find the half-way point between both of the lines on the throttle housing. Make a mark and this will show when the throttle is at half throttle. Divide the spaces up even again until idle, 1/4, 1/2, 3/4, and full throttle positions are known. These lines will be used to quickly find the exact throttle opening while jetting.

Clean the air filter and warm the bike up. Accelerate through the gears until the throttle is at full throttle (a slight uphill is the best place for this). After a few seconds of full throttle running, quickly pull in the clutch and stop the engine (Do not allow the engine to idle or coast to a stop). Remove the spark plug and look at its color. It should be a light tan color (for more info on reading spark plugs click here). If it's white, the air/fuel mixture is too lean and a bigger main jet will have to be installed. If it's black or dark brown, the air/fuel mixture is too rich and a smaller main jet will have to be installed. While changing jets, change them one size at a time, test run after each change, and look at the plug color after each run.

After the main jet has been set, run the bike at half throttle and check the plug color. If it's white, lower the clip on the jet needle to richen the air/fuel mixture. If it's dark brown or black, raise the clip to lean the air/fuel mixture.

The pilot circuit can be adjusted while the bike is idling and then test run. If the engine is running poorly just off of idle, the pilot jet screw can be turned in or out to change the air-fuel mixture. If the screw is in the back of the carburetor, screwing it out will lean the mixture while screwing it in will richen it. If the adjustment screw is in the front of the carburetor, it will be the opposite. If turning the screw between one and two and a half doesn't have any affect, the pilot jet will have to be replaced with either a larger or smaller one. While adjusting the pilot screw, turn it 1/4 turn at a time and test run the bike between adjustments. Adjust the pilot circuit until the motorcycle runs cleanly off of idle with no hesitations or bogs.

Once the jetting is set and the bike is running good, there are many factors that will change the performane of the engine. Altitude, air temperature, and humidity are big factors that will affect how an engine will run. Air density increases as air gets colder. This means that there are more oxygen molecules in the same space when the air is cold. When the temerature drops, the engine will run leaner and more fule will have to be added to compensate. When the air temerature gets warmer, the engine will run richer and less fuel will be needed. An engine that is jetted at 32š fahrenheight may run poorly when the temperature reaches 90š fahrenheight.

Altitude affects jetting since there are less air molecules as altitude increases. A bike that runs good at sea level will run rich at 10,000 ft due to the thinner air.

Humidy is how much moister is in the air. As humidy increases, jetting will be richer. A bike that runs fins in the mornings dry air may run rich as the day goes on and the humidity increases.

Correction factors are sometimes used to find the correct carburetor settings for changing temperatures and altitudes. The chart in fig 8, shows a typical correction factor chart. To use this chart, jet the carburetor and write down the pilot and main jet sizes. Determine the correct air temperature and follow the chart over to the right until the correct elevation is found. Move straight down from this point until the correct coreection factor is found. Using fig 8 as an example, the air temperature is 95š fahrenheight and the altitude is 3200 ft. The correction factor will be 0.92. To find out the correction main and pilot jets, multiple the correction factor and each jet size. A main jet size of 350 would be multiplied by 0.92 and the new main jet size would be a 322. A pilot jet size of 40 would be multiplied by 0.92 and the pilot jet size would be 36.8.

Correction factors can also be used to find the correct settings for the needle jet, jet needle, and air screw. Use the chart from fig 9 and determin the correction factor. Then use the table below to determine what to do with the needle jet, jet needle, and air screw.
 

Email  gassrx2001@yahoo.com

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