How to Calculate Jet Sizes for a Holley Carbby Kelvin Hayes
The size of the jets within your Holley carburetor will control the flow of fuel from the fuel bowl to the combustion chamber. Holley jets range in size from 40 to 99, with the stock jet size differing based on the carburetor model and engine application. Holley manufactures each carburetor with a set of jets that will allow the carburetor to perform well at sea level and around 70 degree air temperature. To calculate the proper jet size for your vehicle, you'll need to know your elevation and outside temperature.
Establish the elevation and temperature the carburetor will be operating within.
Establish the current jet size of the jets within the Holley carburetor.
Adjust the jets one size for every 2,000 foot change in elevation or for every 35 degree change in temperature. For example, your current elevation is 2,000 feet above sea level with a temperature of 70 degrees. You can calculate that the jets should be increased by exactly one size. If the vehicle will be operated at sea level, but the temperature is 35 degrees, you'll need to reduce the size of the jets by 1 size, as indicated by the stamp on the head of the jet.
- Each jet is stamped with its size, so adjustments can be made without measuring.
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