Dellorto, an Italian carburetor manufacturer, measures fuel nozzles differently than other manufacturers due to the unique design and calibration standards they use for their products. While many carburetor manufacturers use a universal numbering system to indicate the size of fuel nozzles, Dellorto uses its own system based on the flow of fuel through the nozzle.

Dellorto's fuel nozzle size is determined by the flow of fuel in grams per hour that can pass through the nozzle when there is a pressure difference of 0.15 bar (approximately 2.18 psi) between the fuel inlet and outlet. The nozzle size number corresponds to this flow value, while other manufacturers often measure the nozzle size by the diameter of the orifice in millimeters.

The main reasons for this difference are:

Dellorto carburetors are designed to provide accurate fuel metering and vaporization for a variety of engine applications. Their unique nozzle metering system allows them to achieve optimal performance and tuning characteristics for different engines.

By using a flow-based system, Dellorto can more precisely adjust the fuel delivery in its carburetors to suit different engines, ensuring a more precise and efficient combustion process.

Dellorto carburetors and their corresponding nozzles are often not interchangeable with other brands' products, which means they need a unique metering system to maintain compatibility within their own product range.

Although the nozzle measurement systems used by Dellorto and other manufacturers may differ, the overall goal is the same: to provide accurate fuel delivery and optimize engine performance. However, this difference in measurement systems can make it difficult to directly compare Dellorto nozzles with those from other manufacturers.