Typical carburetor specs include measurements of 20, 22 and 26 millimeters. Other carburetors are 34 and 35 millimeters. These specs are determined by the bore, or body opening measurements on the carburetor's intake manifold side. Carburetor specs commonly determine their model numbers. For example, a Mikuni VM26 has a 26-millimeter bore.
To determine an engine's necessary carburetor specs, use the engine's size, RPM and volumetric efficiency. Measure the engine's size in cubic inches or centimeters, and approximate the engine's volume efficiency. Stock engines typically have .75 to .85 VE, while mild-built engines generally have .85 to .90 VE.
Use the engine's maximum RPM as shown on the car's tachometer to make sure the carburetor is big enough. Then, enter the engine's measurements into a carburetor CFM Calculator to generate an estimate for the necessary carburetor specs.
A carburetor is simply an open pipe that allows air to flow into the engine's inlet manifold. The pipe's are generally in Venturi form, meaning they widen and narrow alternatively to increase airflow speed into the engine. The Venturi is connected to the throttle valve that rotates to increase or decrease airflow into the engine, maintaining appropriate air-to-fuel ratios in the engine and therefore controlling the car's speed.