Carburettor ranges
Which component acts in which load range
Idle · 0–5 % TPS → pilot circuit
Air flows only through the narrow gap under the closed slide. Fuel comes exclusively through the pilot jet und wird über die pilot/fuel or air screw fine-tuned. The two work in opposite directions: pilot/fuel screw in = leaner, air screw in = richer.
Transition · 5–20 % TPS → throttle slide
The throttle slide starts to lift. Its cutaway (chamfer) and the start of the needle taper set the mixture — this is where the pilot circuit stops having an effect and the main jet has not taken over yet.
Part throttle · 20–60 % TPS → needle jet & jet needle
Slide half open. The mid-range of the jet needle controls the opening in the needle jet. The needle's clip position has the strongest effect on the mixture here.
Full throttle · 60–100 % TPS → main jet
Slide fully raised, needle pulled clear of the needle jet. Now the main jet alone limits fuel flow. Pick the main jet size for full-throttle tuning.
The drawing shows a classic round-slide carburettor. Flat-slide and CV (constant-velocity) carburettors work on the same principle — the load-range mapping applies there too.