An incorrectly jetted bike will display some of the following faults: Uncertain starting, uncertain idling, poor running under load, high fuel consumption, a big, dangerous step in power, or a total lack of power at the transition from low revs to medium revs, hot running (Possibly resulting in a heat seize) or continual fouling of the spark plug. In short, the bike will be a misery to ride.
Unfortunately, getting the jetting right on your bike is a bit of an art. This article will allow you to get the basics under your belt so you can start experimenting and acquiring experience. We are going to assume that you have a Keihin PWK carb on your bike.
When we say the bike is running rich, it means that the motor is burning too much fuel. The engine note will sound “fluffy” rather than crisp and sometimes it will “four stroke” meaning that it misfires every other stroke.
Running lean means that too little fuel is making its way into the motor. Lean running is typically characterized by the motor surging. The revs rise and fall very quickly in a regular rhythm.
To try and uncomplicate the process we divide the rev range into three parts:
Low speed, from twistgrip closed to twistgrip ¼ open, governed by the pilot jet and air screw.
Medium speed, twistgrip ¼ open to ¾ open, managed by the needle.
High speed, twistgrip ¾ open to full open, managed by the main jet.
Further complications. The following factors will change the way that the bike runs:
Altitude. As altitude increases, bike runs richer (because air is thinner/ less air)
Silencer packing. Worn silencer packing will make the bike run leaner
Air cleaner. A clogged air cleaner will make the bike run richer
Power valve. The power valve setting can change jetting either way
Float level. A high float level will make the bike run richer
Temperature of motor. A cold motor will run lean.
Barometric pressure. High pressure will make the bike run lean
Ambient temperature. High temperatures will make the bike run rich
Setting the air screw. The first and last step. Start the bike and bring it up to normal temperature. Let the bike idle. Now start turning the airscrew in and out while listening to the engine note. The airscrew is a small brass screw on the rear left of the carb. You want to find the airscrew position which makes the motor run fastest. Once you have found that position, turn the airscrew fully in, carefully counting the turns until the screw bottoms. Turn it out again to the original position.
Pilot jet. If the best position of the airscrew is over 3 ½ turns out, you will have to install a leaner pilot jet. If the best position is ½ a turn out you will have to go richer on the pilot. The pilot jet is found in the float chamber of the carb and is a small brass screw, removed with a screwdriver. The pilot jet is correct when the airscrew is between 1.5 and 3 turns out. An easy way to access the float chamber is as follows: Loosen the front and rear hose clamps. Remove the cap of the carb with the slide and needle, then twist the carb so that you have access to the four float bowl screws. Undo them and carefully wiggle the float bowl off.
Setting the needle. The first thing to do is to mark the throttle. With the throttle closed, make a mark on the twistgrip body, corresponding to a mark on the twistgrip. Open the throttle full and make a mark on the twistgrip opposite the mark on the body. Now make a mark at ¼ throttle and at ¾ throttle. Get the bike good and warm. and ride off slowly through the gears. Make a mental note of the position of the throttle when the bike is running badly. If the throttle position is between ¼ throttle and ¾ throttle, you will have to change the height of the needle in the slide. Decide whether the bike is running rich or lean. If rich, you will have to lower the needle. If the bike is lean, raise the needle. In some cases you may have to change the needle to a different type. Get advice from your mechanic.
Main Jet. If the bike is running badly at high revs, with the twistgrip more than ¾ open, the main jet will have to be changed. The main jet is situated in the float bowl, a 6mm hex sided brass widget.
Important stuff to remember. The most important thing to remember is that if you jet the bike too lean it will overheat and may seize. If that happens at high speed you will certainly end up on your ear. Be conservative when jetting. Secondly, unless you ride a total exotic, your reputable bike workshop will have a fair idea of the correct jetting. Get advice from them and don’t stray too far from their recommendations. Thirdly, don’t change more than one thing at a time and make notes of the whole process as you go. Finally, the recommendations in your user manual are likely to be so conservative that they are useless, rather get real world advice from experienced mechanics or the internet.