Outboard Motor Carburetor Icing?

 20 Feb 26:

We are reviewing carb icing for aircraft, and it dawned on me, is carb ice a thing for outboard motors? Yes, outboard motors with carburetors are susceptible to carburetor icing, particularly in high-humidity conditions with air temperatures between -5F to +60F. The rapid evaporation of fuel and the low-pressure area created at the throttle valve can lower temperatures enough to turn moisture into ice, causing rough idling, power loss, or engine stalling.

Here's a chart from the FAA

Factors and Symptoms: 

Conditions: High humidity (above 65%) combined with cool, damp air is the most critical factor. 

Low Power Operation: Icing is most likely during trolling, idling, or in low-power, long-descent scenarios, where the throttle is partially closed and the engine generates less heat. 

Symptoms: A gradual drop in RPM, rough running, stalling, or hesitation, especially when transitioning from trolling speeds. Once a motor quits, it will not restart until the ice melts.

Prevention: Running at higher power settings helps prevent ice formation. Using fuel additives like isopropyl alcohol (Heet) can help manage moisture in the fuel system.

While less common than in aircraft due to warmer, enclosed engine covers or more efficient cooling systems, it is a real issue for carbureted outboards in the right conditions, often mistaken for fuel contamination.