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 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, 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: 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.

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 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.

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.

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 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.

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, 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—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—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.