Technique: The air screw and how to adjust it!
A properly adjusted carburetor is essential to ensure the proper functioning of a combustion engine. In these times of injection, throttle bodies and electronic engine control, we thought it necessary to highlight that one screw on your carburetor. Many engines are still equipped with a good old carburetor and it is important to adjust it perfectly. Even more than four-strokes, two-strokes are more sensitive to incorrect adjustment. There are many things to adjust on a carburetor, but today we are talking about the air or mixture screw.
Air screw
In the English language, the air screw is also called “air screw” or “mixture screw”. Without going into detail, this is used to adjust the amount of air/petrol. An engine roughly runs on fourteen parts air to one part gasoline. It runs at its best with this ratio and the mixture screw is its fine adjustment. Before you start adjusting: perfectly adjusting an air screw is only possible if the rest of the carburetor is equipped with the correct parts and corresponding adjustment. This means that your gas factory is mounted with the correct main jet, gas needle, needle seat, float level, air nozzle and throttle slide.
- Make sure the engine is at operating temperature before making any adjustments. Bring the engine up to a higher steady speed and ask a friend to hold the throttle as still as possible at about 1800 rpm. You can also temporarily increase the idle speed on the carburetor.
- As soon as the engine runs at the increased speed, you can turn the air screw (clockwise) until the engine speed drops. Turn the screw in further to the point where the engine is just about to stop. Then you turn the screw in the other direction so that the speed increases again.
- Stop turning the air screw (counterclockwise) when the engine reaches its highest speed. At this point the engine runs best on the aspirated mixture. This means that it runs at its most economical with maximum power. When the engine has reached its highest speed, stop turning the air screw.
- Don't worry if you have to repeat this process a few times to find that peak in the RPM. Once you have found this peak, turn the air screw again to the stop and count the number of turns and write them down. This way you always have the correct adjustment at hand. You adjust an air screw according to the number of outward turns. Manufacturers communicate this in this way in their adjustment data.
- Usually the standard settings will be between half and two and a half turns. If your setting is outside 2,5 rpm, you will need to make the main jet one size smaller. If the speed peaks before you can turn the air screw half a turn, the main jet needs to be one size larger.
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