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The first step is to replace the old spark plug with a new one. Make sure it is the proper plug for your machine, and that it is gapped properly. An improper plug gap will throw off the readings. If the gap is too small, it will not burn properly. If it is too large, it will cause added stress on other ignition components, causing premature failure. You should have fresh fuel at the proper octane level. Most small engines run best with 92-94 octane fuels. It is also important that you have a clean air and fuel filters for the tests.
Plug testing is the easiest way to help you dial in the jetting of the carburetor after adding performance parts and for different altitude or weather changes. The color of the spark plugs is used as an indicator and lets you know what way to start re-jetting the carburetor.
There are three basic ranges that a plug test can be used to check.
3/4-Full throttle, which is controlled primarily by the main jet
1/2 throttle, which is dominated by the taper angle of the needle and the clip position.
1/4 throttle, which is mainly, controlled by the strait diameter of the needle but also the slow circuit.
Testing the Main Jet
Once you have a new plug installed and gapped properly, start the machine and let it warm up. Take the machine for a VERY short ride to bring it up to normal operating temperature. Find an open spot of road and get it up to top gear. Roll on the throttle and hold
it wide open; 2 strokes for 3-5 seconds. 4 strokes 10 - 15 seconds. While you are still wide open you must stop/kill the engine using the kill switch while pulling in the clutch at the same time, all the while still holding the throttle wide open. Let the bike coast to a stop with the clutch still pulled in .Do not let the clutch out until you come to a complete stop.
**** It is very important that you do not let the engine slow down or idle at all. You must do it this way for every test!****
Now you need to remove the spark plug and check its condition. You want to look and take note at the white insulator part of the plug only, not the metal tip or electrode.
The insulator of the spark plug must be a light grayish-tan to light brown color. If the insulator is white, it is too lean, meaning the main jet is too small. If the insulator is black or wet, it is too rich, meaning the main jet is too big.
The steps for checking the other ranges are basically the same. If you want to check the 1/2 throttle range you want to hold it steady at 1/2 throttle in top gear for about 20-30 seconds. The same for 1/4 throttle testing, hold it steady at 1/4 throttle. Pull in the clutch and shut if off at the same time while holding the throttle at a steady speed.

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 effect the mixture at different throttle positions. There are 4 throttle positions that are used to determine what component is effecting 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 start with the slow circuit and work your 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 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 through 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 effect 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 effects 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 diameter of the needle is what affects the mixture at 1/4 throttle. The taper of the needle and the clip position effect 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 settings 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 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 without 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 a 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 RPM's. A rich mixture will rev ok but sputter in the higher RPM ranges

More Too come!!!!!