I especially like the part explaining the different circuits in the carb operation, which I have copied below. He also has nice explanations of mixture, the needle jet, etc.
The Basics
When people talk about jetting a carburetor, tuning a carburetor or "breaking out the brass" they're talking about manipulating the carburetor's 4main circuits to optimize gasoline delivery and therefore engine performance. They might adjust the air screw, adjust the jet needle’s clip position or exchanging the pilot(slow) jet, main jet, throttle valve (slide) or jet needle for one of an appropriate size. A perfectly tuned 2-stroke engine/carburetor delivers a 12.5 to 1 air to fuel ratio
No jet acts independently of the others but rather they work together to deliver gasoline to the engine. They do however target specific throttle openings and have the most effect is that area. See below:
- The air screw is most effective between idle through 1/8 throttle.
- The pilot (slow) jet is most effective between 1/8 through ¼ Throttle
- The slide valve is most effective between 1/8 through ½ Throttle
- The jet needle is most effective between 1/4 through ¾ Throttle.
- The main jet is most effective between 3/4 through wide-open throttle.
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