Zenith Stromberg carburetors surge—especially at steady throttle or light load—when there’s an imbalance in fuel delivery or air intake. Surging usually feels like the engine is repeatedly gaining and losing power without you moving the throttle.
Here are the most common causes of surging in a Zenith Stromberg carburetor:
1. Vacuum Leaks
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A vacuum leak can lean out the air/fuel mixture and cause the engine to "hunt" or surge.
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Check intake manifold gaskets, vacuum hoses, carb-to-manifold base gasket, and brake booster lines.
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Spray carburetor cleaner around suspected leak points while idling—an RPM change indicates a leak.
2. Damaged or Stiff Diaphragm
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The diaphragm controls the movement of the air piston and metering needle.
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If it’s stiff, torn, or incorrectly installed, the needle won't track vacuum changes properly, resulting in inconsistent fueling.
3. Incorrect Mixture Setting
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A mixture that's too lean will often cause surging at light throttle.
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Adjust the mixture screw richer by turning it counterclockwise in small increments.
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You want the smoothest idle without a drop in RPM.
4. Worn or Dirty Needle and Jet
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If the metering needle or jet is worn or has debris buildup, the fuel flow will be inconsistent.
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Remove the piston assembly and clean both thoroughly. Inspect the needle for grooves or distortion.
5. Incorrect or Weak Dashpot Oil
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The dashpot (damper) oil controls the rate at which the air piston moves.
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Use SAE 20 weight oil (e.g., Stromberg or SU carb oil). Thin oil causes the piston to flutter, making mixture control unstable—especially during transitions.
6. Float Level Too Low
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If the float level is set too low, fuel delivery will lag during throttle changes, causing lean surging.
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Remove the carb top and measure the float height. Typically, it should be set between 17–19mm depending on the model.
7. Air Piston Sticking
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The piston must rise and fall freely.
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Check for burrs or carbon on the piston or in the chamber.
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With the engine off, manually lift and release the piston—it should fall smoothly with a faint "thunk."
8. Inconsistent Ignition Timing or Weak Spark
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Surging may not be carb-related at all.
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Check timing, points, dwell, and plug gaps.
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A weak ignition system under load can mimic carburetor problems.
Bottom Line: Surging on a Zenith Stromberg is most often caused by a vacuum leak, lean mixture, or diaphragm/dashpot issues. Systematically inspect each of these areas and adjust accordingly.
