Help with main jet selection
A jet is nothing more than a copper screw with a hole drilled through it. The shape and diameter of the nozzle hole determine the fuel passage. The size of the jet hole is crucial for achieving the right fuel/air mixture for proper engine combustion. If the jet is too large, it will result in a rich mixture, and if it's too small, it will create a lean mixture. Overheating/seizure in 2-stroke engines is often caused by a lean mixture. A carburetor has two adjustment screws: an idle screw and an air screw. The idle screw is used to adjust the idle speed.With the air screw you determine the amount of air that passes.
Main Jet Sizes
Most jet sizes are indicated in hundredths of millimeters. For example, size 70 means 0.7 mm, which is the diameter of the jet hole. However, there are also manufacturers who use different dimensions. You determine the thickness of the jet by measuring the thickness of the screw thread.
When to Replace a Jet?
You only replace a jet when you are making adjustments to your engine. With a 2-stroke engine, you can check the combustion by looking at the color of the spark plug. A good combustion is indicated by a medium brown color. If the combustion is too lean, the spark plug will turn white, indicating the need to enlarge the jet. When an engine is running too rich, you will see soot on the spark plug.
How to Determine the Type of Jet Needed?
First, look at the brand of the carburetor. In some cases, there is only one type of jet, and you simply need to determine the nozzle size. With certain carburetor brands, it's a bit more complex as they use multiple types or thicknesses. In such cases, you need to measure the jet thread using a caliper. The jet size is usually indicated on the nozzle itself. When it is no longer visible, a jet gauge is a perfect solution.
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