Definition
A precisely sized orifice in a carburetor or fuel-injection system that controls the rate at which fuel flows into the induction airstream. The size of the jet's opening determines how much fuel is delivered for a given pressure drop across it, setting the fuel-to-air mixture for a particular operating condition.
Plain English
A small hole of a specific size that controls how fast fuel can pass through it. By choosing the right size hole, the engine gets the correct amount of fuel mixed with the air it is breathing in.
Context Anchor
Seen in piston-engine fuel system, carburetor, and mixture-control discussions.
Derivation
Jet' here comes from the same root as a jet of water -- a stream forced through a narrow opening. 'Metering' means measuring out a controlled amount. Together: a small opening that meters out a measured stream of fuel.
Why Pilots Care
Correct fuel metering prevents engine roughness, power loss, or damage from overly rich or lean mixtures.
Analogy
A metering jet is like the small opening in a spray bottle nozzle: the size of the opening helps control how much liquid can pass through.
Intuition Check
A metering jet is not a jet engine part. Here, jet means a small fuel passage that controls flow.
Example Sentence 1
The mechanic found that a partially clogged metering jet was causing the engine to run lean at cruise power.
Example Sentence 2
A worn metering jet can cause the engine to run lean at high power settings.