Why does my outboard idle rough or surge?

Rough idle / surging at idle · Symptom: Idles unevenly, hunts, or shakes.

Quick answer: Most often: gummed carb idle circuit (after storage), fouled plug / misfiring cylinder, air leak in fuel supply. Start here: Try fresh fuel with stabilizer/cleaner per label. If that doesn't fix it, work through the checks below, and see a marine mechanic if you're unsure.

Likely causes (most to least common)

1. Gummed carb idle circuit (after storage)
2. Fouled plug / misfiring cylinder
3. Air leak in fuel supply
4. Water/stale fuel
5. Idle speed out of spec
6. Blocked idle exhaust relief (some models)

Safe owner checks & fixes

1. Try fresh fuel with stabilizer/cleaner per label.
2. Inspect and replace plugs.
3. Check fuel connections and bulb.
4. Check idle speed per manual.
5. If persists after fuel/plugs, carb cleaning or EFI diagnosis by a mechanic.

Parts / tools

Plugs, fuel filter, fresh fuel, fuel system cleaner.

Call a marine mechanic when

Rough after fresh fuel and plugs (carb or EFI injector service); one cylinder consistently dead; knocking noise. If you're on the water and unsafe, call TowBoatUS/Sea Tow or the Coast Guard (VHF 16).

Sources

• https://cdn.powerequipment.honda.com/marine/pdf/maintenance/Avoiding-Carburetor-Fuel-Problems.pdf
• https://ethanolrfa.org/file/360/Winterization-2014.pdf
• https://yamahaoutboards.com/owner-center/owners-manuals

Values not stated here are model-specific; check your owner's manual.

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