# Carburetor calibration

The catalog contains seven carbureted two-strokes and two carbureted
four-strokes. The 450 cc four-stroke is now carbureted; the 650 cc stays EFI.
The 125 variants use a 3% fixed slide stop plus closed-throttle ignition trim;
the 50/500 singles use 4.5%. The big-block uses 7%.

## Measurement

The `fluidsim_carb_calibration` tool imposes crankshaft RPM with an external
motoring/absorbing dyno, runs 1 s to settle, and averages the following 0.25 s.
Each operating point starts from a fresh warm scenario. At idle, the dyno is then
removed for a further 3 s to check that the engine continues running without
starter assistance or artificial sustaining torque.

| Family | Idle RPM | Low RPM | Mid RPM | Full-throttle RPM |
| --- | ---: | ---: | ---: | ---: |
| Two-strokes | 3000 | 4000 | 6500 | 9000 |
| 450 single | 1400 | 2500 | 4000 | 6000 |
| Big-block | 800 | 1800 | 3000 | 5000 |

Low points use 5%, 10%, 15% and 25% pedal; mid points use 50% and 75%.
The mechanical stop remains in force if pedal demand is lower than it.
AFR acceptance is 11.5?15.5, narrowed to 11.5?14.5 at full throttle. Free-idle
AFR must also stay within 11.5?15.5, with positive running RPM (300?10000).
The fixed slide sets idle airflow. Closed-throttle ignition timing trims speed
toward 3,000 RPM on the 125s, 3,000 RPM on the 50, and 3,000 RPM on the 500.
Four-stroke targets are 1,400 RPM for the 450 and 800 RPM for the big-block.

## Settled idle and carb-outlet AFR

| Scenario | Stop | Unloaded idle RPM | Idle AFR | 15% AFR | 50% AFR | Full AFR |
| --- | ---: | ---: | ---: | ---: | ---: | ---: |
| Racing 125 | 3% | 3603 | 13.49 | 14.56 | 13.56 | 13.27 |
| Straight 125 | 3% | 3754 | 13.48 | 14.70 | 13.32 | 13.27 |
| Long chamber 125 | 3% | 3598 | 13.37 | 14.64 | 13.42 | 13.21 |
| Short chamber 125 | 3% | 3657 | 13.61 | 14.58 | 13.66 | 13.26 |
| Wide chamber 125 | 3% | 3679 | 13.44 | 14.37 | 13.58 | 13.27 |
| 50 two-stroke | 4.5% | 3591 | 14.46 | 14.71 | 13.64 | 12.92 |
| 500 two-stroke | 4.5% | 3128 | 13.63 | 13.23 | 13.85 | 13.26 |
| 450 four-stroke | 4.5% | 1362 | 13.54 | 14.10 | 13.67 | 13.07 |
| Big-block | 7% | 763 | 13.52 | 14.68 | 13.12 | 13.17 |

The previous 125 calibration measured approximately 7.6 at idle, 22.6 at low
pedal and 20.2 at full pedal. The previous big-block reached approximately 28.7
at full pedal. Independent pilot and main jet dimensions plus a continuous,
monotone needle annulus profile correct the off-idle and upper-throttle gaps.
These are effective flow calibrations, not measured commercial jet sizes.

The tool reports `charge_afr` separately: crankcase air/fuel for two-strokes,
intake-plenum air/fuel for four-strokes. Reverse flow, residual oxygen depletion,
wet-wall transients and transported unburned fuel make that differ from fresh
carb-outlet AFR. It is diagnostic rather than the calibration target. Combustion
still consumes only the oxygen and fuel actually present; AFR is not clamped to
hide a bad mixture and no feedback loop forces the target values.

## Reproduction

```powershell
cmake --build --preset windows
ctest --preset windows -R carb_calibration --output-on-failure
./out/build/windows-gcc/fluidsim_carb_calibration.exe --sweep yz-racing
```

The calibration sweep includes all eight throttle positions for every carb scenario,
target idle RPM and AFR, zero pipe positivity repairs, the catalog slide-stop settings,
and transported fueling on the 450. Catalog and two-stroke regressions also
exercise throttle changes, coast-down, clutch/gear operation and finite audio.

[Four-point AFR, charge, and idle measurements](carb_calibration.csv) include charge AFR,
actual idle opening, free-running idle RPM and pipe repair counts.