Coolant-system

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.

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 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. 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. Learn to do it yourself. It's easier than you think. Here is a quick lesson on how to jet your bike.

The Ride and Feel Method

The most basic method 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 mini-bike 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.

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.

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.

Fuel and Oil Mixture Ratio:

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.

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we have kx80 2000 model, it has been ported and bored 20 over and we are having a problem on the lower end of the throttle. everyone states that once the porting is done it will be hard to readjust the throttle to have a quick response, i need some help and wanted to know if going to the slow jetting an d larger jet for it will help on this matter..thanks

got an new banshee, she is run in by now, but I doubt her jetting is correct.
she is real lazy down low on rpm and has a dead spot before before accelerating.
Im at 4500ft above sea level. ideas?

what jettign are you currently running? probably a bit rich if stock jetting @ 4500 feet.

I have an 01 banshee just installed fmf sst pipes stock otherwhise, called alba racing for jet specs they sent a 300 main 32.5 pilot and needle when atv is started it revs to full throtle but runs alright but not great on half choke any sugjestions?

what jetting should i go with for toomey t6's and airbox removed...with clamp on K&N's???????????

ask toomey themselves. They can give you jetting specs that will be damn close to perfect.