BASIC CARBURETION. To be able to properly jet or re-jet a carburetor you must first know how they work. Most ATV and off-road motorcycle carburetors work basically the same way. The two most popular models are the Keihin and Mikuni. These carbs have two primary fuel circuits the slow/pilot circuit, and the main circuit, which have several parts or components that will affect the mixture at different throttle positions. There are 4 throttle positions that are used to determine what component is affecting the mixture, they are idle/off idle,1/4 throttle,1/2 throttle, and 3/4 to full throttle. It is important that you remember that the main fuel circuit is only part of the entire fuel delivery and any changes you make to the slow circuit will affect the main circuit as well, because the slow/pilot circuit is delivering fuel from idle to full throttle. So you must to start with the slow circuit and work you way up to the main circuit.
THE PILOT/SLOW CIRCUIT The pilot/slow circuit has two major components that affect the mixture; they are the pilot/slow jet and the air mixture screw. The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered threw the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.
THE MAIN CIRCUIT This circuit has 4 components that affect the mixture from about 1/4 throttle to wide open/full throttle. These parts are the throttle slide cut-away, jet needle or needle, the needle jet, and the main jet. The throttle cut-away affects the amount of air but can be left alone. The needle primarily controls the mixture from 1/4 throttle to 3/4 throttle. The needle moves up and down inside the needle jet as you open and close the throttle. The strait diameter of the needle is what affects the mixture at 1/4 throttle. The taper of the needle and the clip position affect the mixture at 1/2 throttle up to 3/4 throttle. At 3/4 to full throttle the mixture is controlled mostly by the main jet.
STARTING FROM THE BOTTOM The first throttle range to dial in is the idle/off idle range. Any time there is a weather change or you are riding in a different area you may need to adjust the mixture screw for a clean and crisp engine response. The two main setting are the air mixture screw and the pilot/slow jet. The correct setting is the one that allows a crisp engine response with the air screw about 1 3/4 turns out. If you need to turn the air screw out more than 2 1/2 turns, then it is too rich and you need a smaller pilot jet. If you need only to turn the mixture screw out 1 turn, then it is too lean and you need a larger pilot jet.
1/4 THROTTLE The mixture in this range is mostly controlled by the strait diameter of the needle. A lean mixture will cause the engine to hesitate and a rich mixture will cause it to sputter while keeping it at a steady speed. If there is hesitation then a smaller needle is needed. If it is rich a larger one will need to be installed.
1/2 THROTTLE This range is controlled by the clip position and taper angle of the needle. The correct clip position is often all you need to dial in this range. The correct position is the one that will allow the engine to rev cleanly from 1/4 throttle to 3/4 throttle with out hesitation or sputtering. Raising the needle clip will lean the mixture and lowering the clip will richen the mixture. If you find that you are all the way to the top or all the way to the bottom positions and the jetting is still off ,you may need a different needle jet (if available) or needle with different dimensions.
3/4 TO FULL THROTTLE this range is controlled mostly by the main jet. A correct main jet will allow the engine to rev cleanly all the way until it drops off the power band with out sputtering or hesitation. A lean mixture will have power in the lower RPM s but will get sluggish and/or hesitate in the higher RPMs. A rich mixture will rev ok but sputter in the higher RPM ranges.
THE PILOT/SLOW CIRCUIT The pilot/slow circuit has two major components that affect the mixture; they are the pilot/slow jet and the air mixture screw. The pilot/slow jet has a fixed diameter hole that controls the amount of fuel being delivered threw the slow circuit. The mixture screw is an adjustment screw to control the amount of air being mixed with the fuel from the slow jet.
THE MAIN CIRCUIT This circuit has 4 components that affect the mixture from about 1/4 throttle to wide open/full throttle. These parts are the throttle slide cut-away, jet needle or needle, the needle jet, and the main jet. The throttle cut-away affects the amount of air but can be left alone. The needle primarily controls the mixture from 1/4 throttle to 3/4 throttle. The needle moves up and down inside the needle jet as you open and close the throttle. The strait diameter of the needle is what affects the mixture at 1/4 throttle. The taper of the needle and the clip position affect the mixture at 1/2 throttle up to 3/4 throttle. At 3/4 to full throttle the mixture is controlled mostly by the main jet.
STARTING FROM THE BOTTOM The first throttle range to dial in is the idle/off idle range. Any time there is a weather change or you are riding in a different area you may need to adjust the mixture screw for a clean and crisp engine response. The two main setting are the air mixture screw and the pilot/slow jet. The correct setting is the one that allows a crisp engine response with the air screw about 1 3/4 turns out. If you need to turn the air screw out more than 2 1/2 turns, then it is too rich and you need a smaller pilot jet. If you need only to turn the mixture screw out 1 turn, then it is too lean and you need a larger pilot jet.
1/4 THROTTLE The mixture in this range is mostly controlled by the strait diameter of the needle. A lean mixture will cause the engine to hesitate and a rich mixture will cause it to sputter while keeping it at a steady speed. If there is hesitation then a smaller needle is needed. If it is rich a larger one will need to be installed.
1/2 THROTTLE This range is controlled by the clip position and taper angle of the needle. The correct clip position is often all you need to dial in this range. The correct position is the one that will allow the engine to rev cleanly from 1/4 throttle to 3/4 throttle with out hesitation or sputtering. Raising the needle clip will lean the mixture and lowering the clip will richen the mixture. If you find that you are all the way to the top or all the way to the bottom positions and the jetting is still off ,you may need a different needle jet (if available) or needle with different dimensions.
3/4 TO FULL THROTTLE this range is controlled mostly by the main jet. A correct main jet will allow the engine to rev cleanly all the way until it drops off the power band with out sputtering or hesitation. A lean mixture will have power in the lower RPM s but will get sluggish and/or hesitate in the higher RPMs. A rich mixture will rev ok but sputter in the higher RPM ranges.
