Electronic Control Module (ECM)
According to signal received from O2 sensor, ECM judges, by comparing actual mixture against stoichometric range, whether exhaust gas at the manifold is rich or lean and issues signals to duty solenoids.
Air/fuel ratio is judged to be rich when voltage produced by the O2 sensor is higher than a set specified level. As a result, ECM issues signals to the duty solenoids to by-pass a greater quantity of air into the carburetor. This additional air leans the air/fuel mixture.
Air/fuel ratio is judged to be lean when voltage produced by the O2 sensor is lower than a set specified level. As a result, ECM issues signals to the duty solenoids to by-pass a lesser quantity of air into the carburetor. The elimination of air enriches the air/fuel mixture.