Idle speed drift in a Zenith Stromberg (or any carbureted engine) means the idle RPM gradually increases or decreases on its own, even when you haven’t touched the throttle or mixture screws. This can make tuning frustrating and unstable.
Here are the most common causes:
1. Vacuum Leaks
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Air leaking past gaskets, throttle shafts, or vacuum hoses changes the air/fuel ratio.
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As the engine warms up, gaskets expand, causing intermittent leaks.
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Solution: Spray carb cleaner around intake, manifold, and shaft areas while idling—RPM change = leak.
2. Worn Throttle Shaft or Bushings
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Air sneaks in at the shaft bore when the shaft is worn.
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This creates an air leak that worsens as the engine warms up.
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Solution: Replace bushings or throttle body.
3. Sticking or Binding Air Piston
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In Stromberg carbs, the air piston must move freely to adjust fuel flow.
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If it binds slightly, it alters mixture and idle RPM.
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Solution: Clean piston and bore; verify smooth vertical movement with the engine off.
4. Incorrect or Missing Dashpot Oil
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Thin oil or no oil causes the piston to move too fast or flutter.
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Results in unstable mixture and idle drift.
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Solution: Use SAE 20W or specific carburetor oil.
5. Improper Mixture Adjustment
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If the idle mixture is too lean or rich, the combustion can be erratic, especially during warm-up.
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The mixture must be readjusted after float or needle work.
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Solution: Set mixture after the engine is fully warmed up.
6. Float Level Too High or Low
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A float that sits too high or too low will alter fuel delivery.
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Changes in fuel level affect idle richness or leanness.
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Solution: Check and adjust float height to spec (typically 17–19mm).
7. Ignition Timing Drift
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Weak or unstable ignition timing due to worn distributor advance, coil issues, or loose timing settings can affect idle RPM.
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Solution: Verify timing is steady with a light.
8. Choke Malfunction
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If the choke is sticking or slowly disengaging, idle will change as the engine warms.
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Solution: Inspect manual or automatic choke operation.
9. Engine Temperature-Related Expansion
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As parts heat up (manifold, throttle shaft), they expand. If there's a warped gasket or distorted throttle plate, idle can shift.
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Solution: Ensure proper torque and gasket fitment.
Bottom Line:
Idle drift is usually a combination of vacuum leaks, improper mixture, or mechanical wear. Start with a vacuum test, then check the dashpot oil, throttle shaft condition, and float level.
