How to Tune Your Two Stroke Carburetor, Carburetor Tuning

Carburetor Tuning Explained
carburetor jet range chart, carburetor tuning

Two Stroke Carburetor Tuning Explained

A correctly jetted carburetor can mean the difference between a great day of riding and a frustrating day filled with fouled plug changes. A correctly jetted carburetor makes a huge difference in the torque, mid-range pull, top-end pull, and over-rev of your engine. If your bike isn’t jetted correctly, you will be sure to gain improved performance by getting it right. A well jetted pilot circuit can mean the difference between having to clutch the bike out of a turn or not. The needle can make all the difference in the world for the power of the machine in most situations, as it controls the throttle range that most riders spend most of their time using. A correctly sized main jet could mean the difference between being able to rev out high enough to not have to shift one more time at the end of the straight, or the power falling flat on top and requiring you to make that extra shift.

Is your bike sputtering or going flat at a particular throttle opening? Are you fouling plugs?

Every bike is different, the only way to know what jetting changes you will need to make is by reading the plug and seat of the pants testing. Jetting for two strokes is heavily affected by the weather. A simple increase in humidity can throw your jetting off. If you want your two stroke to perform at its peak, jetting is a useful skill to learn.

To begin the process, you will want to start with the pilot jet. The pilot jet affects the entire throttle range. When you are at full throttle, the main jet is the primary fuel metering device, but the pilot is still delivering fuel, adding to the total amount of fuel that your engine is receiving.

Make sure you have a clean air filter, a fresh plug and fresh fuel. 

We will begin by gently turning the air/fuel mixture screw all the way in until it stops, don’t lock it down, just turn it until it stops turning easily. You don’t want the needle to break off inside your throttle body. Then back it out 1.5 turns, start the engine, and turn the idle screw in until you get a slightly fast idle, to keep the engine idling. Turn the air screw slowly in, and then out, and see how it affects the rpms. You are trying to find the point where the idle is fastest.

Now is the time to determine if you have the correct pilot installed in your carburetor The air/fuel mixture screw position determines this for you. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet. Once you have determined the correct pilot jet size, and tuned the air screw for the fastest idle, it’s time to tune the air screw for the best throttle response. Make sure the bike is at full operating temperature. Set the idle back down and ride the bike, using closed-to-1/4 throttle transitions. Turn the air screw slightly in either direction until you find the point that gives you the best response when cracking the throttle open. Most bikes are sensitive to changes as small as 1/8 of a turn. The air screw is not a set-it-and-leave-it adjustment. You have to constantly re-adjust the air screw to compensate for changing temperature and humidity. An air screw setting that is perfect in the cool morning air will likely be too rich in the afternoon heat.

After establishing the correct pilot jet you will need to set the needle. Ride the bike at 1/8 to half throttle. You can mark your grip with tape if it helps. If the bike bogs and goes flat for a second before responding to throttle, it is lean. Lower the clip one notch at a time until the engine picks up smoothly. If the bike sounds crackly when giving it throttle, it is rich. Raise the clip one notch until it runs cleanly. This is the way to test the needle, you will know instantly when you have it right.

Once you have the needle position set, we will move our attention to the main jet. The main jet affects from 1/2 to full throttle. The best way to test it is to do a throttle-chop test. With the bike fully warmed up, find a long straight, and install a fresh plug. Start the engine, and do a full-throttle run down the straight to second or third gear. As soon as the bike tops out, pull the clutch in, and kill the engine and roll to a stop. Remove the plug, and look deep down inside the threads, at the base of the insulator. If it is white or gray, the main is too lean. If it is dark brown or black, the main is too rich. The correct color is a medium-dark mocha brown or tan. See the example below.

In time you will learn the difference in sound and feel between rich and lean and all your riding buddy’s will be asking you to help them jet their bikes.

One last thing, make sure your engine is in good mechanical condition. If your engine has a worn top-end, fix it first. Re-jetting a tired engine is a waste of time. Check that your reads seal properly, and be certain your silencer is in good working order and doesn’t need to be re-packed.

 

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