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How to Understand Carburetor Jet Sizing


The jets on a carburetor meter the amount of fuel that enters the throttle bores of the carburetor where it mixes with incoming air. If the engine shows performance problems such as stalling or sluggish acceleration, you’ll need to change the jet sizing. Jets are threaded and have a small orifice through their center that varies based on the size of the jet. For example, a jet may be stamped (on its head) with the number 30. This indicates that the jet’s size is 30 mm. Installing a smaller size jet will reduce fuel flow, which is ideal for high-altitude conditions where air is limited, where a larger jet size increases fuel flow for oxygen-rich, low altitudes. With this knowledge, you’ll be able to understand how to correctly change the jets to regain the engine performance levels that were lost due to improper jetting.

Jet Sizing For Altitude Changes

Step 1

Remove the jets and insert a smaller/larger set based on the vehicles operating altitude. When the operating altitude of the engine will be 2000 feet higher than your normal altitude,…