Before you can identify the default mainjet you must know actual pressure, temperature and humidity. After completing this step proceed to:
2. Determine mainjet
There are three options available (I, II and III) to obtain this information:

  1. Find a weather station (see below) that measures pressure, temperature and humidity. (usually most accurate and convenient method)
  2. Get all weather data online (NOAA, Weather Underground, etc.), from a cell phone text message service or by listening to Weather Radio or ATIS, which is affiliated with your local airport. (usually least accurate method)
  3. Same as option II but obtain temperature and humidity information from a less complex weather station; one that measures temperature and humidity only.
  4. Note: regarding option II and III- If you choose II or III and don't live near sea level, correct barometric pressure (mean sea level pressure or altimeter setting) to actual or absolute pressure using the Jetdial shown in the picture below as follows: Align standard pressure 29.92 inHg with altitude and barometric pressure will be aligned with actual or absolute pressure. Example:
  5. Note: regarding option II- Confirm that the data from the official weather station is recent.
  6. Note: regarding option I and III- To ensure that you read the actual or absolute pressure (not mean sea level pressure, altimeter setting or barometric pressure), set altitude to sea level if your weather station asks for any altitude information. In addition, do not place your weather station in direct sunlight and be sure that it is acclimatized to the weather conditions.
  1. Note: regarding option II and III- Barometric pressure (mean sea level pressure or altimeter setting) at sea level is equal to actual or absolute pressure. Therefore, if the track is near sea level there is no need for any pressure correction determination.
  2. Note: regarding option III- It's not problematic if you don't live close to the next official weather station. Compared to temperature and humidity, pressure does not vary significantly over a relatively wide region on any given day, as long as there are no storms, hurricanes or tornados in the area. Examine the current barometric pressure map from Wunderground: There's a pressure difference of 4 hPa from line to line. The same mainjet, however, is valid for approximately a 50 hPa pressure difference! (In other words: If the same temperature and humidity was present in San Francisco and New York, two identical karts, one at each location, would most likely utilize the same mainjet).
  3. Note: regarding option I and III- Hand-held weather stations that can measure (barometric) pressure, temperature and humidity can be found online (search for weather station, barometer) or even at your local hardware store for less than $100. Many hand-held weather stations measure temperature and humidity only, and they can generally be purchased for less than $30. These less complex weather stations can even be found at some smaller stores and even CVS pharmacies.