Stromberg AAV Troubleshooting

This guide covers common performance issues and mechanical failures specific to the Stromberg AAV 2-barrel carburetor. Whether you are dealing with a vintage Buick, Chrysler, or Cadillac, these troubleshooting steps will help isolate the cause.

1. Fuel Flooding or Leaking from Top

If fuel is spilling out of the air horn or the engine is "loading up" at idle, check the following:

• Dirt in the Needle and Seat: This is the most common cause. Even a microscopic piece of rust from the fuel line can prevent the needle from seating.

• Incorrect Float Level: If the float is set too high, the fuel bowl will overfill. Check your specific application specs for the correct "dry" float setting.

• Heavy or Sunk Float: Inspect the brass floats for pinhole leaks. If they contain fuel (shake them to hear a slosh), they will stay at the bottom of the bowl and never shut off the fuel flow.

• Excessive Fuel Pressure: Modern electric fuel pumps often put out 7–10 PSI. The Stromberg AAV generally prefers 3 to 4.5 PSI. Anything higher will blow past the needle and seat. You may have to add a fuel regulator.

2. Hesitation or "Flat Spot" on Acceleration

If the engine stumbles when you step on the gas, the issue is likely in the Accelerator Pump Circuit:

• Worn Pump Leather: The leather cup on the accelerator pump plunger can dry out or shrink, especially if the carb sat empty for a long time.

  • Pro Tip: Soak a dry leather cup in light oil for 10-15 minutes to see if it expands before replacing.

• Check Ball Not Seating: There are usually two check balls in the pump circuit (inlet and outlet). If the discharge check ball is gummed up or missing, the fuel will just pump back into the bowl instead of out the shooters.

• Clogged Discharge Nozzles: Remove the air horn and verify that fuel "squirts" immediately when the throttle is moved. If not, the small brass nozzles may be clogged with varnish.

3. Rough Idle or Stalling at Stoplights

• Vacuum Leaks: Check the base gasket and the throttle shaft bushings. Old Strombergs often develop wear in the throttle body where the shaft passes through, allowing "unmeasured" air to enter. Spray carb clean on the shaft ends. If RPM changes then there is too much wear. Take it to a machinist to get bushings installed. 

• Idle Mixture Adjustment: Ensure the two idle mixture screws are adjusted evenly. Start at 1.5 turns out and adjust for the highest vacuum reading.

• Clogged Idle Tubes: The internal idle tubes have very small orifices. If the engine won't idle unless the choke is partially closed, these tubes are likely restricted.

4. Rich Running (Black Smoke / Fouled Plugs)

If the engine runs rich even after adjusting the idle screws, look at the Power Circuit:

• Stuck Power Piston: As discussed, if the vacuum piston is stuck in the "down" position, the power valve stays open, dumping extra fuel at all times.

• Leaking Power Valve: If the power valve (at the bottom of the bowl) isn't seating properly, fuel will bypass the main jets.

• Choke Not Opening: Ensure the thermostatic spring (if equipped) or the manual cable is allowing the choke plate to stand perfectly vertical when the engine is warm.

5. Hard Starting (Cold vs. Hot)

• Cold Start: Check the choke piston (unloader). If it’s carbon-fouled, it may not allow the choke to snap shut or open slightly once the engine fires. Located on the inside of the thermostat housing.

• Hot Start (Percolation): In modern summer-blend fuels, the fuel in the bowl can literally "boil." Ensure you have a thick heat-insulator gasket between the carburetor and the manifold. Make sure fuel lines are away from the exhaust manifold.

Quick Reference Table: Symptom vs. Cause