2 stroke Engine Tuning Guide

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http://blogs-aj.blogspot.com/2009/03/2-stroke-engine-tuning-guide.html





2 Stroke Engine Tuning Guide

   
  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.



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 hole-shot 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 learnt from a great  mechanic. You don't need any fancy tools, just the ability to make  observations while you ride.


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  old 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.



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 to 1 (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. 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.



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.



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 levels: When the float level is too low, the jetting  will seem to  be too lean and 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 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.


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.

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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 must 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  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 a RAD   gauge will show 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 jets 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- 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- 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.