The deceleration control system is designed to maintain a balance air/fuel mixture during periods of engine deceleration. Although the components used vary in some years, the basic theory remains the same: To more thoroughly burn or dilute the initial rich mixture formed when throttle is suddenly closed, and to smooth out the transition to a lean mixture by enriching the mixture slightly after the throttle has closed. Although the process may seem contradictory, they act in sequence to provide an overall ideal mixture.
The 1972 49 States and Canada Mazdas use a vacuum control valve, an accelerator switch, a servo diaphragm, and a set of spark retard points in the distributor. The vacuum control valve controls spark retard, through the ignition points, and throttle opening through the servo diaphragm, and a set of spark retard points in the distributor. The vacuum control valve controls spark retard, through the ignition points, and throttle opening through the servo diaphragm. During deceleration, the vacuum control valve applies vacuum to the servo diaphragm, slightly opening the primary throttle plate to admit additional fuel to the lean mixture. At the same time, it activates the retard points through a vacuum switch. At idle, the accelerator switch activates the retard points; above idle speed, the accelerator switch is off and the standard point set is used.
1972 Mazdas sold in California, and all 1973–74 Mazdas use an anti-afterburn valve, and a coasting richer valve on the carburetor which is controlled by three switches: speedometer, accelerator, and clutch. 1975–76 Mazdas use the same system, but the clutch switch is not used. The anti-afterburn valve has a diaphragm controlled engine vacuum. During deceleration, the diaphragm lifts allow the air pump to inject air into the intake manifold. This dilutes the incoming rich mixture, preventing detonation in the exhaust system, which would occur if the injected air from the air pump were to burn this rich mixture in the exhaust manifold. The coasting richer valve acts to add additional fuel to the lean intake mixture as soon as the anti-afterburn valve has shut off. The speed, accelerator and clutch switches must be closed to allow the coasting richer valve to operate. The accelerator switch closes when the accelerator pedal is released. The speedometer switch closes when the truck speed is above 17–23 mph. The clutch switch, used from 1972–74, is closed when the clutch pedal is released.
1977–78 1,796cc 49 States and Canada Mazdas use an air by-pass valve, a carburetor dashpot, and a throttle opener system comprised of a servo diaphragm connected to the throttle and a vacuum control valve. This is essentially the same as the 1972 49 States model. The air by-pass valve prevents afterburn in the exhaust by shutting off air to the exhaust manifold from the air pump during deceleration. The dashpot holds the throttle open slightly for an instant during sudden deceleration. The throttle opener system is the same as that used in the 1972 49 States models.
1977–78 California Mazdas use an anti-afterburn valve and a throttle opener system. The anti-afterburn valve is the same as that used from 1973–76, and the throttle opener is the same as that used in 1977–78 49 States models.
1979 and later 1,970cc 49 States models use an anti-afterburn valve and a throttle positioner. California models have an air by-pass valve and a throttle positioner. Canadian models have an air by-pass valve, a dashpot, and a throttle positioner.
On 1974 models with automatic transmission, a kick-down control system is used. Regardless of the gear selected, the transmission will not go above Second gear when the choke knob is pulled out.