Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Feedback Carburetor System: Operation
The "X" system is the main feedback system. It controls air/fuel mixture by controlling the opening of air control valve B according to the amount of vacuum at the diaphragm chamber of the valve. The frequency solenoid B is modulated by the control unit, and controls the amount of vacuum to air control valve B. On Prelude models, manifold vacuum applied at frequency solenoid valve B is constantly modulated by the constant vacuum valve.
Mixture is more finely controlled by the "M" system, based on engine load. The diaphragm chamber of air control valve A receives the same carburetor vacuum as the EGR control valves A and B, and air is supplied to the intake manifold in proportion to the carburetor intake air volume. Frequency solenoid valve A receives signals from the oxygen sensor through the control unit and opens the valve when air/fuel mixture is richer than pre-set (stochiometric) mixture.
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.