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Prelude BA4 Carburettor Check

Twin side-draft CV carburettors · PGM-CARB · Fuel system
Osaka Classic Garage · We hunt. You win.

Hey, Kevin here 👋🏽 This tool is meant to help you with your carb problem, or at least give you a better understanding of it. Feedback or bug reports are welcome at @osakaclassicgarage.

No prior knowledge needed

What does a carburettor actually do?

An engine burns a mixture of air and fuel. The carburettor is the component that mixes the two – purely mechanically, using nothing but airflow, floats and tiny holes (jets). The Prelude has two of them side by side. The control unit (ECU) helps out with small electric valves.

Below you can operate the carb yourself: ignition ON, give it gas, engine cold or warm – and see what happens inside.

The recipe
14.7 kgair
+
1 kgfuel

= ideal mixture (“λ = 1”). More fuel means rich, less means lean.

Try it out

Carburettor simulator

Air cleaner Air in Engine stopped Choke valve Piston rises with the airflow Needle in jet Throttle valve (gas pedal) Float chamber Float Float valve Power valve Fuel tank P Pump off Cut-off solenoid de-energized: closed Vent valve to air cleaner Charcoal canister
Ignition switch
Engine speed0
Pumpoff
Mixture–
VentingCharcoal canister
The path of the fuel

Seven stations from the tank to the cylinder

You’ll also find the numbers on the system map. “Show on the map” zooms straight there.

    Rich or lean?

    The mixture ratio decides everything

    too leanideal (λ = 1)too rich
    Too little fuel

    Bucking, flat spot when accelerating, popping back through the intake, engine runs hot, stalls at idle.

    Just right

    Smooth idle, clean throttle response, O2 sensor swings between 0.4 and 0.6 V.

    Too much fuel

    Black smoke, fuel smell, sooty plugs, high consumption, afterfire, catalytic converter runs hot.

    Glossary

    The technical terms, explained simply

    How the fuel system works together

    The whole system in one picture: from the tank at the rear to the engine. Pick a situation above (e.g. “Cranking cold”) and you’ll see what happens. Every part can be tapped.

    1 · Pick a situationThe buttons below, e.g. “Warm idle” or “Deceleration”.
    2 · Read the coloursGreen = active / energized, orange = partial, faded = off. Moving yellow line = fuel is flowing here.
    3 · ZoomMouse wheel or two fingers. Drag to pan. Full screen with ⛶ at the top right.
    4 · Tap a partOn the right (below on a phone) you’ll find a plain-language explanation of what it does.
    Layers
    100 %
    Mouse wheel / two fingers = zoom · Drag = pan Air path (side-draft) Air cleaner TA-Sensor Air leak valve Air Bleed B Air jet controller B11 A4 Choke valve Choke opener Thermovalve Cranking leak valve #18 #26 ST · BLU/RED Vacuum piston Jet needle / needle jet Piston control valve Piston solenoid Chk C Air chamber #28 #20 B14 Throttle valve Thermowax Fast idle cam A/C idle boost B10 Engine Intake manifold MAP TW O2 sensor A6/A7 A3 B1 Float chamber 16 ± 1 mm Float Float valve Crossover pipe → 2nd carburettor Main jets Slow jet Cut-off solenoid Mixture screw B4 Power valve #14 Accel. pump Fuel tank Pump Rear filter Front filter 17.7–22.6 kPa Cut-off relay No. 12 Term. 4 ignition pulse (BLU) → Ignition coil Term. 1 YEL/BLK BLK/YEL Outer vent Charcoal canister Inner vent Throttle controller #6 B12 B13 B9 Purge · B5 Air inj. PGM-CARB ECU Passenger footwell LED = blink code Control Box (8P) Vacuum switches etc. 1 2 3 4 5 6 7
    active / energized partial position inactive moving line = circuit is flowing

    Source: Honda Prelude shop manual, Chapter 11 “Fuel and Emissions (Carbureted Engine)”, pages 11-1 to 11-122 (US version, 49 States / California): fuel supply, carburettor, PGM-CARB, air intake and emission control including the evaporative emission system.

    EU vehicles: European BA4 carburettor models may differ in emission and control parts (O2 sensor, catalytic converter, EGR, individual solenoid valves). Before working on your own car, compare the fitted parts with the map.

    Working on the fuel system: no smoking, no open flames, drain fuel into an approved container. Use the propane method only with a propane enrichment kit, outdoors or in a well-ventilated area.