Inspect Flame Arresters to Save Lives

Most boaters driving gasoline powered inboard boats know that there is something on top of the carburetor. They probably think it's to keep bugs, loose clothing and other airborne objects from being sucked into the engine. Those who are more knowledgeable may know it is a backfire flame arrester, but don't know how important it is. Some might even think about taking it off to "let the engine breathe" better and increase speed, even one-tenth of a mile per hour.

On one boat some years back, with a V-6 stern drive, the arrester had been replaced with an automotive paper element air cleaner. The owner still had the sack from the auto parts store with the sales slip. There was an engine compartment explosion and ensuing fire. A little girl, who was asleep on cushions under the fore deck, never made it out. They sued the boat manufacturer, the stern drive manufacturer and even GM [General Motors], which made the basic engine. They didn't get a dime.

Backfire flame arrestors serve a very important safety function. All gasoline engines with carburetors tend to vent fuel vapor after shutdown. If shut down from high speed, they are hot and will boil off some of the fuel in the float bowl. This vapor goes into the engine compartment and will linger for a while unless taken out by the ventilation blower. The vapor itself does not harm, as long as there is no source of ignition present. Sources of ignition can be electrical or from a backfire in the engine intake system when it is restarted. Most of us have heard a backfire. On a boat, flames spouting out of the carburetor can ignite that fuel vapor in the engine compartment.

The operating principle of the flame arrester is to cool and quench the flame front by forcing it to pass between some closely spaced metal elements that act as a heat sink. Since the engine must take in air without too much restriction, many elements are required with many small spaces between, hence the typical arrester design of a circular shape that is six or more inches in diameter and an inch or more in height. Some form of delayed or late ignition takes place in one of the cylinders at a time when the intake valve is open and the resulting flame ignites the fuel and air vapor in the intake manifold. The pressure created has to go somewhere, and that's back out through the carburetor or intake system. With a working flame arrester, only the pressure is released and none of the flame gets out.

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