Information about the links or groups of links shown below.
DRIVE LINE-
Chain Care-

Types of Chains
There are only a couple of different types of chains on the market. Basically its broken down into O-Ring Chains and Standard Chains. The difference is that the O-Ring chain is low maintenance and very durable because it is internally lubricated. The original grease from the factory is held inside the rollers by rubber o rings on each side of the link. These chains are especially good for woods riders because they offer superior dirt and grime protection while preventing water from getting into the chain itself. The sacrifice made however is that the O-Ring chains do rob horsepower from the bike because the O-Rings rub and cause friction. For most woods riders the trade off is well worth the cost in money and horsepower loss. I have a friend who has had the same O-ring chain on his bike for 3 seasons and has NEVER had to adjust his chain.
The X-Ring chain is basically the same as an O-ring chain in design, but the rings have 4 grooves in them and are supposed to reduce the friction or loss of power because less actual rubber is in contact with the chain itself.

Adjustment
When adjusting your chain remember one thing if you remember anything, DO NOT adjust your chain tight. The chain on a dirt bike requires there to be slack in it because as the suspension goes up the chain usually tightens. The number one reason you don’t want this condition is that the chain will pull I on the front sprocket and possibly break the case of your engine if it doesn’t just wear out the countershaft bearing and seal. If you think about coming off a jump with all of that energy transferring from the suspension to the countershaft through the chain and right to the front sprocket you can get an idea of why you don’t want this condition.

How to adjust most chains on most dirt bikes. First place your bike with the rear wheel off the ground and loosen the two adjustment blots, there should be a lock nut and a bolt. In most cases these are seized to the aluminum swingarm and need a spray of penetrating oil to get them to move (be careful – you don’t want to break off the adjuster bolt inside the swingarm). To loosen them you need to loosen the locking nut counter clock wise and back it off a couple of turns. Now you need to loosen the axle nut (only loosen). Once you loosen the axle nut you can turn the adjuster bolts out together to push the axle back and tighten your chain. Use your owners manual to find out what the slack on the drive chain is supposed to be, My Bike is between 1.5” and 1.9” of slack. Yours may be different.
Watch the marks on the sides of the swingarm to make sure you are aligned correctly. If your rear wheel is slightly off you will get headshake. Once you have adjusted your chain, use a torque wrench and tighten the axle nut. DO NOT OVER TIGHTEN THE NUT. Again, follow manufacturer specifications here. If you over tighten your axle nut you can cause your axle bearings to heat up and fail. Once you have it tight double check that the axle didn’t move and tighten the adjuster bolt locking nuts.
After you adjust your chain its best to have someone check it while you jounce the suspension to make sure it doesn’t tighten up. Some KTM’s are especially hard to adjust and the best way is to remove the rear shock bolt, and lift the swingarm till you find the tightest point, about half way, then adjust the chain from there. Once you learn your bike you’ll know right where it needs to be.

Cleaning and care-
Special care needs to be taken when dealing with O-Ring chains, number one rule is do not use WD 40 as this is the fastest way to ruin your rubber o ring parts. WD-40 is a great cleaner for non o ring chains but it destroys and caused rubber to dry out. For an o ring chain the best bet is to use hot soapy water and a nylon or plastic brush, never ever ever use a metal brush on an o ring chain. This will get in and destroy the soft o rings. Also, never use high pressure to clean an o ring chain as the water will force its way past the o rings and into the chain itself. This of coarse will trap the water inside causing the links to rust and corrode from the inside out. You are never supposed to use any kind of harsh solvents on an o ring chain, most of my frinds use mineral spirits on their chains which works very well.

For non o ring chains you can use pretty much whatever you want to, I often do use WD40 between rides to clean it off. This has 2 benefits that its acts as a cleaner and it displaces water which will helps get moisture out of the middle of the link rollers. But all in all the best way to clean your chain is to remove the master clip and clean it in solvent or mineral spirits. If you haven’t done this before stand back and be amazed at the amount of dirt and metal that will come out of your chain. The metal and dirt that is in there will wear at your rollers, and where the links meet each other, forcing the links to spread and prematurely wear out your chain. Pulling off your chain is much easier than it sounds as long as you have the master link, and this is the best and most effective way to keep your drive chain in good condition for happy riding.

Once your chain is dry apply a generous coating of o ring safe chain lube( I have been using the Bell Ray Super Clean Chain Lube on my bikes and have been very happy with it). If you apply the chain lube the at least the night before the lube will dry and be less tacky which will attract less dirt, Sidewinder makes a fully synthectic chain lube that is supposed to dry completely which will attract even less dirt.

Chain Rollers
The Chain rollers are probably one of the things you never think about when you are servicing your bike, They don’t seem to do much and really don’t effect performance. But none the less you need to check them out when you are cleaning your bike. Make sure the rollers turn freely, if they don’t you may be able to remove them and lubricate them. Otherwise you can buy them for around Ten Dollars.

Chain Slide
Heres a basic item you need to have on your bike, If you have purchased a bike and it doesn’t have one of these you need to get one, they are about and keep your chain fron grinding your swingarm in half.

Chain Guide


Sprockets-
Sprockets are amazingly the best way to tailor your bike to your riding needs, if you need more or less speed, you can change your sprockets to fit where you ride. Typically 1 tooth on the front is equal to 4 teeth on the rear sprocket. Front sprockets are cheaper and easier to change for the most part than the rear sprockets. You can have a few front sprockets and really have the ability to change your bike to your riding needs.

Types
There are many types of sprockets out there in the world of dirt bikes, Everything from Aluminum to Titanium and every metal between. But unless you’re racing motocross you probabley don’t need what the salespeople will try to sell you. The major companies make aluminum sprockets and Renthal is well known for making high quality aluminum sprockets and are stocked by most motorcycle shops. However,,, if you have aluminum sprockets you will find that the teeth will bend over and wear our about 6 times as fast as a steel sprocket.
In my personal opinion I think Sidewinder www.sidewinder.com makes the best sprockets and chains on the market. I know of more professional racers use Sidewinder

Ratios

Inspection and care

Wheels and tires-

Tires

Air pressure

Spokes

Wheels

Hubs-
-inspection and care
-bearings



SUSPENSION-
Motorcycle suspension has changed drastically over the past two decades. Years ago it was simple, you have two rear shocks and a pair of forks. Today suspension is the most complex thing on your bike. If it is set up correctly you can handle nearly any type of terrain you encounter. The forks should be able to glide over chop and rough areas while being able to absorb the hardest hits you will throw at it, they allow you to corner the bike without washing out or cutting in, and can save you from pending disaster. The rear end is able to handle chop coming into a corner, roosting through a corner, and landing from a huge jump. The suspension on todays machines is a modern marvel of complex valving and geometry. If you set up your bike correctly you will learn what its really capable of. In this section I will break down suspension as best as I can and give you intrustions on how to best set up your bike for YOU.
Swingarm-

How it works

Linkage

Care and maintenance

Steering head-

How it works

Care and maintenance

Rear Shock-

How it works

Oil Change

Bearings

http://www.geocities.com/jmurray04/shockrebuild

Front Forks-

How they work

Maintenance
Just like any other part on your bike you need to take care of your forks, these are not maintenance free items and need to be cared for after each ride.
One thing to do is to make sure you release the air pressure from inside your forks with the air bleed screws. As pressure builds up in the forks the ride will become harsher and you risk blowing out your fork seals. The air in your forks will mix with the oil and foam your oil, you should release the pressure in your forks, let them sit overnight, and release the air the next morning before you go ride. Just like a can of soda pop, when there is pressure the air bubble can not expand and raise to the surface. Same thing happens in your forks.

Clean your seals, you want to clean dirt and grime from under the dust seals on your forks. Cleaning your dust seals will help your forks seals to stay clean which will help them live a long and happy life. You also want to watch for oil dribbling out of your forks, if you see more than just a super thin film you need to have you fork seals changed. Fork seals are about to .
Its not a bad idea to have your fork oil changed once a year

Oil Change

Seals and bushings




Suspension Setup and tuning-

Your bikes suspension

(Pro Action guide)

http://www.racebikesite.com/motorcycle-modifications.htm
Suspension Set-Up
Before diving into the complexities of your bikes suspension, you should know what the lingo is. Below are some suspension terms you should become familiar with.


Sag or Static Sag – The amount the bike compresses between fully toped out and fully compressed, with the rider and all rider gear on the bike.
Free Sag – This is the amount the bike will settle under its own weight with no rider on board. You should have roughly 1mm-5mm of free sag.
Damping - Damping is determined by the speed of the fluid through the shock or forks, not the distance of travel. To help understand this better you should know the term Valving which is any component in the shock or fork (usually holes, check valves or shims) used to restrict the flow of oil travel as it is forced up or down. Compression Damping – Controls the initial impact (speed of compression) of the suspension. Compression Damping helps keep the spring from bottoming out or having to much travel. Rebound Damping Controls the rate (speed of extension) at which the suspension extends after being compressed.
Preload – This simply means applying a force to the spring. In other words, it is the amount the spring is compressed from its original (non-compressed) state. This is adjusted obviously with a pre-load adjuster. THIS WILL NOT MAKE THE SPRING RATE STRONGER OR WEAKER! Think of preload as a fine tuning of the ride hieght.
Sag or Static Sag
One of the most common terms you will here in the pits concerning suspension is "Sag" or "Static Sag." This is one of the most important things you need to do to your bike before heading out on the track. Static sag will help determine if you have the correct-rate springs on your bike. This process will require a couple of people.
Setting up the Rear End
1. When measuring the rear sag, use the very end of the swing arm for a point of measurement and a point on the frame that is totally vertical to the point on the swing arm. Extend the suspension completely and measure the distance from the swing arm to the point on the frame. This measurement is L1
2. Get on the bike and have someone push down on the rear end about 1" and let it extend slowly. When it stops, measure the distance using the same two points as measured in step 1. This measurement is L2.
3. Lift up on the rear end about 1" and let it down slowly. Measure between the two points again, this will be L3
Static Sag = L1 – [ (L2 + L3) / 2 ]
Adjust the preload with whatever type shock your bike has. If you have too Much, sag you need more preload. If you have too little sag you need less preload. You should have somewhere between 1" to 1½ " or, 25mm-30mm of sag.
Setting the Font End
Measuring front-end sag is about the same as the rear, but you will need to take seal drag into account.
1. Extend the fork completely and measure from the dust seal (where the large and small parts or the shock meet) to the bottom of the triple clamp (the steering component that is clamed to the forks) This is L1.
2. Get on the bike and have someone push down on the front end and let it extend slowly. When it stops, measure the distance between the same two points. This is L2.
3. Lift up on the front end and let it down slowly. When it stops, measure the distance again, which will be L3.
4. Use the formula above again to calculate the static sag: Due to the drag of the seals, you may get different measurements if the process is done over again. If you want to be more accurate, I suggest going through these steps three times and then take the average.
To adjust the sag, use the preload adjusters located at the top of the forks, or vary the length of the preload spacers inside the fork. The sag distance is similar to the rear…typically 1" to 1 ½ " or 25mm-30mm.
When setting the sag, you may want park next to a wall so that you can help support the bike when sitting on it, while the other person picks up or pushes down on the suspension. Make sure the bike is level (not resting on a wheel stand) when setting your sag. Do not bounce up or down on the bike when taking these measurements. You may also consider taking your measurements using metric measurements as it is easier to calculate.
Ride Height
Ride height at both ends is critical to the proper handling of your bike. As a rule, the more rear ride height you add, the quicker the bike will turn, and the more you add in the front, the slower it will turn. Note also that the slower a bike turns due to these changes makes it more stable. Racers typically look for a quicker, less stable set-up in their bikes.
Helpful Hints:
1. Note that by making any changes to your bike will consequently alter other parts of your bike’s geometry. Make small changes to one section at a time.
2. Making the wheel base smaller by bringing up the rear wheel will quicken steering and put more weight transfer on the rear wheel. This can be beneficial in decreasing wheel spin but will also make the bike less stable.
3. Remember that the "Preload" adjusters will not make the suspension stiffer or softer! The preload is simply how much the springs are compressed. Think of Preload adjusters as Ride Height adjusters.
4. Buy a book on suspensions and have an experienced racer help you with setting up your bike. You will find that what you think is the problem might actually be something totally different.


Steering Dampner

Levers and Controls - adjustments

Throttle- How it works

-Care and maintenance

Clutch

Front Brake

Rear Brake

Cable routing

Handlebars
-triple clamps

Footpegs-





Brakes-

Bleeding

Pads

Rotors

Calipers

Master cylinder

Exhaust Pipe

How a 2 stroke exhaust works

Silencer

Batting

Expansion Pipe

Dents

Clutch-

How it works

Adjustment

Clutch Basket

Clutch springs

Clutch frictions and steels

Clutch replacement
<
The Dreaded Clutch Job

By Mike Hobbs
--------------------------------------------------------------------------------
The dreaded clutch job...

It's not that a clutch job on a dirt bike is difficult; in fact it's kind of easy. When it comes right down to it, it's just one of those things you put off. Why? Well, there are tons of reasons for not getting around to it and these are mine:

My clutch adjustment is all the way out but the clutch is not slipping yet.
The garage is a mess and it would take too long to clean it in order to get the job done right.
If I do it now before the clutch is toast, it will have to be replaced sooner, and it has been raining for two weeks and it will be at least two weeks before it stops, so there is plenty of time to get it done. Right?
Let's look at these excuses logically. Waiting for the clutch to fry would probably cost more then the price of friction disks or clutch springs. My 16-year-son has a girlfriend and is always asking for money and with slick negotiation skills and ten bucks the 16-year old will clean the garage. It is raining and I can't ride anyway, I should just as well do it now. That about does it for the excuses so, let's get to work!

Before you tear into your bike, you have to make sure you actually need to work on the clutch. If while accelerating the clutch slips, you probably need new friction disks and/or clutch springs. On the other hand if your clutch is adjusted all the way out at both ends, it's possible your clutch cable is stretched. If you think the cable might be the culprit call your local motorcycle shop and see if they have a new cable in stock for your machine. From personal experience, while on the phone with them be sure to check that they have everything you might need to complete the job. Depending on their answers to your tough questions, you can now head over to the dealer. If they have the cable, be sure to match it up with yours to see if it is stretched or not. If the cable is fine, just get new friction disks and/or clutch springs and head back home for some clutch repair work. Be sure not to stop by the bar on the way home, this would probably give you time to come up with more excuses.


Detail of Clutch.
How to get it done...

Drain the oil from your crankcase.
Remove the bolts securing the clutch cover. It's best you use the correct size socket to do this. If you don't have the correct size socket and/or you can't find the clutch cover. Put the oil drain plug back in where you took it from and add fresh oil where you add oil to your motorcycle. This is not a complete loss on your part. At least you've changed your oil without the risk of damaging anything that might be important.

After removing the bolts, place them on the ground in the same pattern you took them out.
After removing the bolts, place them on the ground in the same pattern you took them out. That way you can be sure they go back into the same hole(s) they came out of. (Yes! I learned this trick the hard way.)
If you figured out where the clutch plates are and located the correct socket. You can consider yourself a mechanic and continue with our instructions.
On the face of your clutch plates you should see a circular pattern of bolts. These bolts need to be removed.

Remove the bolts, springs, and pressure plate.
The springs on the bolts you just removed might be worn out. Refer to your owner's manual for the "free length". Free length is the measurement from end-to-end on the spring and you should use calipers to check this. If you don't have calipers your local motorcycle shop should and would be happy to check this for you.
Remove the pressure plate.

Set them down in order with the side that was facing the engine up.
Pull the round plates and friction disks from where you removed the bolts in the circular pattern. Don't mix them up, as they need to be in a specific pattern.
Set them down in order with the side that was facing the engine up.
Remove the first disk and set it aside from the others.
Dip a new friction disk in oil and place it over the clutch boss. (This would be the same place you took the disks and plates from.) In case you are still unsure what a clutch boss and clutch housing look like, we have provided a picture.

Detail of clutch boss.
Then place a clutch plate (thin metal) on top of the friction disk that you already put over the clutch boss. Now put an oil dipped friction plate over the clutch plate. Get the idea? Oil each disk and plate before putting them in. Then install them in the following order: friction disk, clutch plate, friction disk, clutch plate repeating this until you have no extra circular items left. Setting aside the old friction disks of course.
While putting the disks and plates back in you will notice that there are groves in the clutch housing and clutch boss. Align the friction disks so they go into the groves. Be sure to check that they are all flush against the clutch housing.

Tighten pressure plate bolts with torque wrench to factory specifications.
Align the pressure plate over the clutch boss and put back the bolts and springs you took out - securing the clutch plates and friction disks. Now refer to your owners' manual for the correct torque (pounds per square inch). If you don't have an owner's manual send Super Hunky an email asking him to provide you with this information. He loves these types of questions. (This is a joke people.) If you don't have a manual and/or a torque wrench borrow or buy them! Save yourself the headache of putting everything back together, just to take it apart again because you strip the bolts out or something is not right.
If you didn't damage the gasket pulling the cover off in Step 2, put the cover back on. If you did damage the gasket, run down to your local motorcycle shop and get a new one. Trying to "rig" a broken gasket makes you a hillbilly and not a mechanic. I don't make the rules; I just try to enforce them.
Because you have the bolts already set aside in the order they were removed, putting them back in is a snap. While putting the bolts back in be sure to tighten them in a crisscross pattern.

Be sure to check your manual for any "tips". For example: on the WR 400 you need to line these goofy things up. This concludes our program on doing a clutch job on a dirt bike. Don't forget to put oil back in the bike when you're done! Jeezzz, do we need to ride the thing for you too? ;-)

Mike Hobbs


ENGINE-

Power Valve

Top End Rebuilding

Reeds

Jetting
By Eric Gore, a real good guide to jetting 2 stroke engines.
for more info about jetting by Steve Fortner - click here

Intro

Carburetor tuning has the greatest effect on engine performance. When a
motorcycle manufacturer builds a bike, they usually install jets in the carb
that are too rich. The manufacturers sell the same model worldwide, so they
couldn't afford to install different jets in the carb to suit all the
different climates and types of fuel. In addition to the climate and fuel,
the manufacturer would also have to consider many other factors, such as the
terrain and type of riding. And then there is the most important jetting
consideration, the rider.

When I worked as a mechanic, I was in charge of jetting the bike over the
course of the day. During morning practice sessions, the track was usually
muddy and the air temperature was at its lowest point. I had to jet the bike
rich for practice because the air density was greater and the mud put more
of a load on the engine. Then I had to watch the rider and the bike perform
on different sections of the track. I would go to the obstacle on the track
that presented the greatest load on the bike, typically an uphill straight
section. I'd listen to my engine and watch the rider. I'd listen for pinging
or knocking noises or excessive smoke from the pipe. I would watch to see if
the rider had to fan the clutch a lot and how my bike pulled in comparison
to others. Getting feedback from the rider is difficult because they are
concentrating on riding not the bike's performance. At a pro national there
is one practice session, followed by a series of qualifiers and eventually
two race motos. The time spacing of the riding sessions over the course of
the day was such that I had to compensate the jetting two or three times.
Otherwise, the bike would either seize from being too lean in the morning or
run too rich for the second moto.

Race mechanics have different techniques for carb jetting. These techniques
range from asking other mechanics what jets they are running to using
precise measuring gauges to monitor the engine performance. In motocross
races, where most of the riders are of equal skill levels, a holeshot in the
start can mean the difference between a place on the podium and 30 minutes
of roost in your face! The difference in horsepower between the bike that
gets the holeshot and the bike that brings up the back of the pack may only
be a few ponies! The race mechanic can give his rider an awesome advantage
if he carefully monitors the carb jetting.

This section will give you insight into the carb tuning process, from
diagnosing mechanical problems that mimic poor jetting to tuning tools such
as gauges. It will also give you tips on a jetting method that I've
developed called the "ride-and-feel" method," which I consider to be the
best method It's a technique that I teach to all the riders I've worked
with. You don't need any fancy tools, just the ability to make observations
while you ride.

The Differences in Two-Stroke and Four-Stroke Carbs

The difference between a two-stroke and four-stroke engine is intake
velocity. Two-stroke engines have lower velocity so the needle jet has a
half-moon shaped hood protruding into the venturi to produce a low pressure
area that aids in drawing the fuel up through the needle jet. Four-stroke
carbs need to atomize the fuel more so than a two-stroke carb because so
much of the fuel shears along the intake port and separates from the mixture
stream. Four-stroke carbs have more jets and finer adjustment screws, plus
they usually are equipped with an accelerator pump. A typical state of the
art four-stroke carb is the Kehin CR.

The latest trend in two-stroke carbs features a pump that sprays fuel into
the venturi from 1/4th to 3/4th throttles. In the past, carb manufacturers
made jet needles that attempted to compensate for the natural lean condition
of the mid-range but that compromised the jetting at full throttle. The
auxiliary pumps are powered by electricity supplied by the alternator (about
5 watts) and controlled by either a throttle position or an rpm sensor.

Identification Guide to Popular Carb Types

On two-stroke engines, several different model carbs have been used over the
years, but there are basically two big carb manufacturers. Kehin and Mikuni
are two popular brands of Japanese carbs used on nearly every dirt bike.

Kehin has several different models. The most popular ones are the PJ, PWK,
and PWM. The PJ is used on Honda CR125, 250, and 500 models from 1985-1997
The slide is oval shaped and there are no additional pumps, its just a
simple carb. In fact it's so simple that the choke and idle screw share the
same jet. The PWK was the next step up from the PJ. The PWK has a crescent
shaped slide and a separate idle circuit from the choke. The PWK is used on
Kawasaki KX125, 250, and 500 models from 1990-97. The latest version of the
PWK features a pump to supply extra fuel in the mid-range. The PWM is
similar to the older PWK (no pump) and the overall length is shorter.

Mikuni has several different model carbs too. The original model VM had a
round slide. There are many different parts available including needle jets
of different diameters and jet needles with different taper angles and
diameters. The next model was the TMX, which became available in 1987. It
was a flat-slide carb, which offered a greater peak flow rate. The TMX was
revised several times, becoming smaller with fewer parts. The TMS carb
introduced in 1992 had no main or pilot jet. The slide and jet needle
handled all the jetting. That carb worked great on 250cc bikes but never
became popular. The PM is the latest Mikuni model. It features an oval
crescent shaped slide and a very short body. That carb comes standard on
Yamaha YZ125 and 250 1998 and newer models.

Carburetor Parts and Function

A carburetor is a device that enables fuel to mix with air in a precise
ratio while being throttled over a wide range. Jets are calibrated orifices
that take the form of parts such as pilot/slow jets, pilot air screw,
throttle valve/slide, jet needle, needle jet/spray-bar, air jet, and main
jet. Fuel jets have matching air jets, and these jets are available in many
sizes to fine-tune the air-fuel mixture to the optimum ratio for a
two-stroke engine, which is 12.5: 1.

Fuel Jets, Air Jets, and Throttle Positions

Three circuits control the air: the air-screw, the throttle slide, and the
air jet. Four circuits control the fuel: the pilot/slow jet, the
spray-bar/needle jet, the jet needle, and the main jet. The different air
and fuel circuits affect the carb jetting for the different throttle-opening
positions, as follows:

Closed to 1/8 throttle-air screw and pilot/slow jet

1/8 to 1/4 throttle-air-screw, pilot/slow jet, and throttle slide

1/4 to 1/2 throttle-throttle slide and jet needle

1/2 to full open-jet needle, spray-bar/needle jet, main jet, and air jet

(Note: On many modern carbs the spray-bar/needle jet and air jets are
fixed-diameter passages in the carburetor body and cannot be altered.)

Basic Carb Service

Nobody likes to fiddle with a carb if they don't have to. Wedged in between
the engine and frame with tubes, cables, and wires sprouting out like
spaghetti, carbs are a pain to work on. Carbs require cleaning just like
anything else, and some careful observations can save you big money in the
long run. Start by pressure washing the bike, especially around the bottom
of the carb where roost from the tires and oil from the chain accumulate.
Take care when removing the carb, it's easy to damage the cable. Its better
to remove the sub-frame so as to enable unrestricted access to the carb.
This will also make it easier to route the vent hoses in their proper
positions too. When you remove the carb look at the vent hoses. Are they
melted from heat or clogged with mud? If so that can cause a vapor-locking
problem in the float bowl and make the engine bog.

Remove the top of the carb and disconnect the cable from the slide. Is the
cable frayed or kinked? Is the rubber dust cover missing? If so then replace
the cable. Now remove the float bowl, jet baffle (white plastic shroud
around main jet), float and fuel inlet needle, and the air-screw. Shake the
floats and listen for fluid that may have seeped inside. If so replace the
floats otherwise the engine might suffer from constant fuel flooding. Check
the fuel inlet needle. It has a Viton rubber tip and occasionally fuel
additives and dirt damage the tip. Also check the spring-loaded plunger on
the opposite end of the tip. If the spring doesn't push the plunger all the
way out then replace it. Check the air-screw, there should be a spring and
o-ring on the end of the needle. The spring provides tension to keep the
air-screw from vibrating outward and the o-ring seals out dirt and water
from entering the pilot circuit. Next check the bell mouth of the carb. Look
for the two holes at the bottom of the bell mouth. The one in the center is
the air passage for the needle jet and the other hole offset from center is
the air passage for the pilot circuit. It's typical for those passages to
get clogged with dirt and air filter oil. That would cause the engine to run
rough because without a steady stream of air to mix with and atomize the
fuel, raw fuel droplets make the jetting seem rich.

Once the carb is basically stripped down (pilot/slow and main jet still in
place) you can flush the passages. Get an aerosol can of brake or carb
cleaner from an auto parts store. Make sure you get the type with the small
diameter plastic tube that attaches to the spray tip. Direct the tip into
the airscrew passage. When you spray the cleaner you should see it flow out
the pilot/slow jet and the air passage in the bell mouth. Next spray through
the pilot/slow jet, look for flow through a tiny passage located between the
venturi and the intake spigot. Spraying cleaner through these passages
insures that the low speed air and fuel circuits are open and free flowing.
The last area to flush with the carb cleaner is the slide bore and slide.
Dirt tends to trap there, causing the mating surfaces to develop scratches
that could cause the throttle to stick!

Just a small amount of water and dirt can get trapped in the tiny passages
of the carb and cause havoc with jetting or even engine damage. How often
should you service the carb? When it gets dirty! For example if you ride in
muddy wet conditions you should at least check the vent hose. If the riding
conditions are dusty and your air filter is covered with dirt, then it's a
good idea to do a basic carb servicing.

Mechanical Problems

The process of jetting-changing air or fuel jets in order to fine-tune
engines' performance-is very simple. Jetting becomes complicated because
mechanical problems sometimes mimic improper jetting. This causes you to
waste time and money trying to correct the problem with expensive carburetor
jets.

Before you ever attempt to jet a carb, make sure the engine doesn't have any
of the problems in the following list. If you are in the process of jetting
a carb and you are stumped with a chronic problem, use this section as a
guide to enlightenment!

Crankcase air leaks-Air leaks can occur at the cylinder base, reed valve, or
the magneto seal. Air leaks make the throttle response sluggish and may
produce a pinging sound. That sound occurs when the air-fuel mixture is too
lean.

Crankcase oil leaks-The right-side crankcase seal is submerged in the
transmission oil. When this seal becomes worn out, oil can leak into the
crankcase. The oil is transferred up to the combustion chamber and burned
with the air-fuel mixture. The oil causes the spark plug to carbon-foul.
This mechanical problem makes the jetting seem to be too rich.

Coolant-system leaks-Coolant systems leaks commonly occur at the
cylinder-head gasket. When the coolant leaks into the combustion chamber, it
pollutes the air-fuel mixture and causes a misfire or popping sound at the
exhaust pipe. Check the engine's coolant level frequently. Hondas and
Kawasakis have characteristic coolant leaks because they use steel head
gaskets. Yamahas and Suzukis use O-rings to seal the head and cylinder.
Coolant-system leaks lower the engine's peak horsepower. It makes the engine
run as if the air-fuel mixture is too rich.

Carbon-seized exhaust valves-The exhaust valves sometimes become
carbon-seized in the full-open position. This mechanical problem can make
the engine run flat at low rpm and make the slow-speed jetting seem lean.
The carbon can be removed from the exhaust valves with oven cleaner. Clean
the exhaust valves whenever you replace the piston and rings.

Blown silencer-When the fiberglass packing material blows out of the
silencer, excess turbulence forms in the silencer and the turbulence causes
a restriction in the exhaust system. This restriction makes the engine run
flat at high rpm.

Broken reed-valve petals-The petals of the reed-valve can crack or shatter
when the engine is revved too high. This mechanical problem makes the engine
difficult to start and can also have a loss of torque. Expert rider should
switch to carbon fiber reed petals because they resist breaking at high rpm.
Novice riders should use dual-stage fiberglass reeds (Aktive or Boyesen).
These types of reed petals provide an increase in torque.

Weak spark-When the ignition coils deteriorate, the engine performance will
become erratic. Normally, the engine will develop a high-rpm misfire
problem. Check the condition of the coils with a multimeter.

Clogged carburetor vent hoses-When the carburetor vent hoses get clogged
with dirt or pinched closed, the jetting will seem to be too lean, so the
engine will run sluggish. Always check the condition of your carburetor vent
hoses. Make sure there is no mud in the hoses and that the hoses are not
pinched between the suspension linkage.

Carburetor float level-When the float level is too low, the jetting will
seem to be too lean, so the engine performance will be sluggish. When the
float level is too high, the jetting will seem to be too rich.

Worn carburetor fuel-inlet needle-When the fuel-inlet needle wears out,
excess fuel enters the float bowl and travels up the slow jet and into the
engine. This makes the carb jetting seem to be too rich. Replace the
fuel-inlet needle and seat every two years.

Jetting Shouldn't Be Scary!

Jetting is the process of making adjustments to the air and fuel jet sizes
in order to fine tune the carburation to suit the load demands on the engine
and make the power delivery consistent and optimum. Too much anxiety is
placed on jetting. Most people just want to call me on the phone and ask
what jets they should put in their carb. That's an impossible question
because that the big dirt bike magazines attempt to answer just to increase
readership. People get confused because they read jetting specs in a
magazine, put those jets in their bike and seize the engine. Any quoted
jetting in this book is just a baseline. Most magazines don't list
parameters for their jetting specs like; Brand new bike running with VP C-12
fuel with Silkolene oil mixed at 30:1 and a NGK 8 spark plug, ridden by a
really slow lard-ass editor twisting the throttle on a hard-packed track.
Some part numbers and jet sizes are given in the Tuning Tips section for
models that definitely need certain jets in order to get the bike near the
baseline. There is an old saying that says you can fish for a man and feed
him for a day or teach him to fish and enable him to feed himself for life.
Here is a quick lesson on how to jet your dirt bike.

The Ride and Feel Method

The most basic method of determining correct carburetor jetting is "ride and
feel." This method requires you to determine if the carburetor tuning is too
rich or too lean by the sound and feel of the engine. The first step is to
mark the throttle body in 1/4-throttle increments, from closed to full open.
Then, this method requires that you ride the motorcycle on a flat, circular
course. To check the carb jetting for throttle positions up to 1/2 throttle,
ride the motorcycle in second or third gear. Roll on the throttle slowly
from 1/4 to 1/2 open. If the engine is slow to respond and bogs (engine
makes a booooowah sound) then the carb jetting is too lean. You can verify
lean jetting by engaging the carb's choke to the halfway position. This will
make the air-fuel mixture richer and the engine should respond better. If
the carb jetting is too rich, then the engine will make a crackling sound;
the exhaust smoke will be excessive and the engine will run as if the choke
is engaged. Careful engagement of the choke can help you determine if the
jetting is rich or lean. Another important tip is to just change the jets
one increment at a time, either rich or lean, until the engine runs better.
Most people are afraid to change a jet because they think that the engine
will be in danger of seizing. Believe me, one jet size won't make your
engine seize but it could be the difference between running bad and running
acceptable.

To check the jetting for throttle positions from 1/2 to full open, ride the
motorcycle in third and fourth gear. (You may need to increase the diameter
of the circular riding course for riding in the higher gears.) Check the
jetting in the same manner as listed above. The carb jets that affect the
jetting from 1/2 to full throttle are the jet-needle, main jet, power jet
(electronic carbs) and the air jet (on four-strokes).

If you want to take this technique out to the racetrack, you can test the
pilot/slow jet when accelerating out of tight hairpin turns, the needle clip
position on sweeper turns and short straits, and test the main jet on the
big uphill or long straits. Of course be careful if you try to use the choke
technique because you could lose control when riding one handed.

Jetting for Riding Techniques

Certain types of riders require jetting to compliment their technique. For
example beginner minibike riders will need slightly richer jetting on the
pilot/slow jet and the needle clip position to mellow the powerband and make
it easier to ride. Conversely desert racers who hold the throttle wide open
for long periods of time need rich main jets to compensate for the high
load.

The Weather Makes The Biggest Difference!

The weather can have a profound affect on the carb jetting because of the
changes in air density. When the air density increases, you will need to
richen the air-fuel mixture to compensate. When the air density decreases,
you will need lean-out the air-fuel mixture leaner to compensate. Use the
following as a guide to correcting your jetting when the weather changes:

Air temperature-When the air temperature increases, the air density becomes
lower. This will make the air-fuel mixture richer. You must select jet sizes
with a lower number to compensate for the lower air density. When the
barometric pressure decreases, the opposite effect occurs.

Humidity-When the percentage of humidity in the air increases, the engine
draws in a lower percentage of oxygen during each revolution because the
water molecules (humidity) take the place of oxygen molecules in a given
volume of air. High humidity will make the air-fuel mixture richer, so you
should change to smaller jets.

Altitude-In general, the higher the altitude the lower the air density. When
riding at racetracks that are at high altitude, you should change to smaller
jets and increase the engine's compression ratio to compensate for the lower
air density.

Track Conditions and Load

The conditions of the terrain and the soil have a great affect on jetting
because of the load on the engine. Obstacles like big hills, sand, and mud
place a greater load on the engine that requires more fuel and typically
richer jetting. In motocross, track conditions tend to change over the
course of the day. Typically in the morning the air temperature is cooler
and the soil wetter requiring richer jetting. In the afternoon when the
temperature rises and the track dries out, leaner jetting is needed in order
to keep the engine running at peak performance. Other changes for mud and
sand riding might include changing to a lower final-drive ratio (rear
sprocket with more teeth) to reduce the load on the engine and help prevent
it from overheating. Advancing the ignition timing will make the engine more
responsive at low to middle rpm.

Fuel and Oil Mixture Ratios

When we talk about the "fuel" in the air-fuel mixture for a two-stroke engine, we are really talking about a mixture of fuel and oil. If you richen the pre-mix ratio (20:1 as opposed to 30:1) there is more oil and less fuel in the same volume of liquid, which effectively leans the air-fuel ratio. And this fact gives the clever tuner one more tool to use when the correct jet is not available or when none of the standard jets are exactly right. You can richen the jetting by slightly reducing the pre-mix ratio (less oil). You can lean the jetting by increasing the pre-mix ratio (more oil). The best part is that changes in the pre-mix ratio affect the jetting over the entire throttle-opening range, but the changes in ratio must be small to prevent excess wear from lack of lubricating oil or fouled plugs from too much oil.
Pre-mix oils are formulated for a fairly narrow range of pre-mix ratios. You should examine the oil bottle for the oil manufacturer's suggestion on the pre-mix ratio. All production two-stroke dirt bikes have a sticker on the rear fender suggesting that you set the pre-mix ratio to 20:1 That sticker is put there for legal purposes. Always refer to the oil manufacturer's suggestion on pre-mix ratios. In general, small-displacement engines require a richer pre-mix ratio than do large-displacement engines because smaller engines have a higher peak rpm than larger engines. The higher the engine revs, the more lubrication it requires.

Tuning Gauges
There are three types of gauges that professional tuners use to aid carb jetting:
1. Relative-air-density (RAD) gauge
2. Air-fuel (AF) ratio meter
3. Exhaust-gas-temperature (EGT) gauge

The following is a description of how each gauge functions and their advantages.
RAD gaugeÑThis is the best gauge for dirt bikes because of the convenience. The gauge is no good unless you get the jetting perfect once. The RAD gauge provides you with an indication of how much the air density changes, helping you compensate for the affects of changes in the air temperature, altitude, and barometric pressure. The gauge is calibrated in percentage points. Once you set the jetting with the ride and feel method, you can set the calibration screw on the gauge so the needle is pointing to 100 percent. When the air density changes, the RAD gauge will show the relative percent of change. Using a calculator you can multiply the percentage change shown on the RAD gauge by the jet size and determine the corrected jet size for the air density. The pilot/slow and main jet have number sizes that correlate with the RAD gauge, but the needle clip position can only be estimated. Normally for every two main jet increments, the needle clip must be adjusted one notch.
AF ratio meterÑThe AF meter measures the percentage of oxygen in the exhaust gasses, and displays the approximate air-fuel ratio of the carb. The gauge displays AF ratios from 10-16:1 The optimum AF ratio for a two-stroke engine is 12:1. The AF gauge utilizes a lambda sensor that is inserted into the center of the exhaust stream, approximately six inches from the piston in the header pipe of a four-stroke and in the baffle cone of a two-stroke engine. A permanent female pipe fitting (1/4in.) must be welded to the side of the exhaust pipe in order to fasten the sensor. The weld-on fitting set-up is also used on the temperature gauges, and the fitting can be plugged with a 1/4in. male pipe fitting when the gauge is not in use. This gauge is ideal for four-stroke engines.
EGT gaugeÑThe EGT gauge measures the temperature of the gasses in the exhaust pipe by means of a temperature probe fastened into the exhaust pipe, six inches from the piston. This type of gauge enables you to tune the carb jetting and the pipe together, taking advantage of the fact that exhaust pipes are designed with a precise temperature in mind.
An exhaust pipe is designed to return a compression wave to the combustion chamber just before the exhaust port closes. Most pipes are designed for a peak temperature of 1,200 degrees Fahrenheit. Most dirt bikes are jetted too rich, which prevents the exhaust gasses from reaching their design temperature, so power output suffers. Sometimes just leaning the main jet and the needle-clip position makes a dramatic difference.
Digitron is the most popular brand of EGT gauge. It measures both EGT and rpm. This gauge is designed for go-kart racing so its not suited for wet weather conditions. It is designed to mount on the handlebars. That way the rider can focus in on it. Once you have performed the baseline jetting, send the rider out on the bike with the EGT. The rider observes the EGT to give you feedback on the necessary jetting changes. Once the jetting is dialed, we use the tachometer to check the peak rpm of the engine on the longest straight of the racetrack. For example, if the peak rpm exceeds the point of the engine's power-peak rpm, then change the rear sprocket to a higher final-drive ratio (rear sprocket with fewer teeth) until the rpm drops into the target range. An EGT gauge is ideal for dirt track bikes and go-karts, where peak rpm temperature is critical.

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