Carburetor Theory and Tuning

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http://twostrokemotocross.com/2009/12/carburetor-theory-and-tuning/



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
 Adjusting the Carb…

 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.

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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  Keihin 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
 Keihin PWK Carburetor.

 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,  and it’s 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.

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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.)
 

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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. It’s 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

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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, 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.
 Check these for leaks.

 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 Kawasaki’s have characteristic coolant leaks  because they use steel head gaskets. Yamahas and Suzuki’s 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.
 Make sure the packing in your silencer has not been blown out. It makes a difference.

 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. 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.

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Jetting Shouldn’t Be Scary!
 Make sure you have a good selection of jets.

 Jetting is the process of making adjustments to the air and fuel jet  sizes in order to fine tune the carburetion 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
 Carburetor animation.

 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.

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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 power  band 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 to 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—generallly, 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.

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Fuel and Oil Mixture Ratios
 The simple, quick and easy way to get your pre-mix ratio correct. A Ratio Rite measuring cup.

 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.
 Relative Air Density Guage from Tanner Racing

 RAD gauge—this 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 meter from Innovate Motorsports

 AF ratio meter—The 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 gauge by Digitron

 EGT gauge—The 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 it’s 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.
 The article above is all about two stroke carburetor tuning and yet  was found on a web site devoted to Street Bikes. Go figure! This is the  site that I found it on http://hondanighthawks.net/carb16.htm
 

Content from the “Motocross & Off-Road Performance Handbook” by Eric Gorr