The Keihin Jetting Calculator helps optimize the fuel-air mixture in carbureted engines, which is crucial for performance, efficiency, and engine longevity. This tool is particularly useful for tuning motorcycles, ATVs, and other small engines with Keihin carburetors.

## Inputs

**Current Temperature (T)**: The current ambient temperature in degrees Fahrenheit (°F).**Current Altitude (A)**: The current altitude in feet.**Current Humidity (H)**: The current humidity percentage.**Current Barometric Pressure (BP)**: The current barometric pressure in inches of mercury (in Hg).**Original Jet Size (J)**: The size of the jet currently installed in the carburetor.

## Formulas and Calculations

## 1. Calculate Air Density Correction Factor (CF)

The correction factor accounts for changes in air density due to temperature, altitude, humidity, and barometric pressure.

[ \text{CF} = \left(\frac{BP}{29.92}\right) \times \left(\frac{460 + 60}{460 + T}\right) \times \left(1 – \frac{H \times 0.004}{100}\right) ]

where:

- ( BP ) = Current barometric pressure (in Hg)
- ( T ) = Current temperature (°F)
- ( H ) = Current humidity (%)

## 2. Adjust for Altitude

Altitude affects air density, and the correction factor for altitude can be calculated as:

[ \text{CF}_A = \left(1 – \frac{A}{1000 \times 100}\right) ]

where:

- ( A ) = Altitude (feet)

## 3. Calculate Corrected Jet Size

The corrected jet size is calculated by multiplying the original jet size by the combined correction factors.

[ J_{\text{corrected}} = J \times \text{CF} \times \text{CF}_A ]

where:

- ( J ) = Original jet size
- ( \text{CF} ) = Air density correction factor
- ( \text{CF}_A ) = Altitude correction factor

## Example Calculation

**Inputs:**

- Current Temperature (T): 85°F
- Current Altitude (A): 2000 feet
- Current Humidity (H): 50%
- Current Barometric Pressure (BP): 29.5 in Hg
- Original Jet Size (J): 150

**Calculate Air Density Correction Factor (CF):**

[ \text{CF} = \left(\frac{29.5}{29.92}\right) \times \left(\frac{460 + 60}{460 + 85}\right) \times \left(1 – \frac{50 \times 0.004}{100}\right) ]

[ \text{CF} = \left(0.986\right) \times \left(0.971\right) \times \left(1 – 0.02\right) ]

[ \text{CF} = 0.986 \times 0.971 \times 0.98 \approx 0.94 ]

**Adjust for Altitude:**

[ \text{CF}_A = \left(1 – \frac{2000}{1000 \times 100}\right) = 1 – 0.02 = 0.98 ]

**Calculate Corrected Jet Size:**

[ J_{\text{corrected}} = 150 \times 0.94 \times 0.98 ]

[ J_{\text{corrected}} = 150 \times 0.9212 \approx 138.18 ]

## Summary

For an engine with an original jet size of 150, at an altitude of 2000 feet, temperature of 85°F, humidity of 50%, and barometric pressure of 29.5 in Hg, the corrected jet size is approximately 138. This adjustment ensures optimal engine performance and efficiency by accounting for environmental changes.

## Table of Relevant Information

Input | Value | Unit |
---|---|---|

Current Temperature | 85 | °F |

Current Altitude | 2000 | feet |

Current Humidity | 50 | % |

Current Barometric Pressure | 29.5 | in Hg |

Original Jet Size | 150 | (no units) |

Corrected Jet Size | 138.18 | (no units) |

## Conclusion

The Keihin Jetting Calculator helps tune engines for optimal performance by adjusting the jet size based on current temperature, altitude, humidity, and barometric pressure. This ensures the correct fuel-air mixture, enhancing engine efficiency and longevity.