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.