How to Inspect and Clean a Backfire Flame Arrestor

Look up the flame arrestor in your owner's manual. Makers specify how often to clean it and what to clean it with, and some arrestors have a coating or a built-in filter element that certain solvents or pressure can damage. Follow those instructions where they differ from this guide.

With the engine off and cool, open the hatch and run the bilge blower for several minutes. Sniff for fuel. Remove the key and disconnect the battery if you will be working near the starter or other wiring.

Take photos of the arrestor, its cover, and any hoses connected to it. On many engines one or two crankcase vent hoses (from the valve covers) plug into the arrestor or its cover. Label them so they go back to the same place.

Remove the decorative cover if your engine has one, then the nut, bolt or clamp that holds the arrestor down. Disconnect the vent hoses and lift the arrestor straight off. Right away, cover the carburetor or throttle body opening with a clean rag or tape so nothing can fall into the engine. A dropped nut or washer in there can do serious damage.

Hold it up to a light and look at the grid all the way around. Replace it if you find:

• Holes, tears, crushed or separated sections in the grid or between the plates
• Corrosion that has eaten into the metal
• A cracked or warped base, or a base that no longer seals against the carburetor
• Missing markings showing it meets SAE J1928, UL 1111 or USCG approval

Light dirt and oily film are normal and can be cleaned. Heavy oil soaking means the crankcase ventilation system may need attention; mention it to your mechanic.

Use the cleaner your engine maker recommends. If the manual gives no specific product, Boating Magazine's general advice is a mild solvent or soap and water, with a commercial degreaser for heavy grime followed by a freshwater rinse. Swish the arrestor in the solution and work the grid with a soft nylon brush. Never use a wire brush or poke the grid with tools, and never use gasoline, which leaves a flammable residue and is dangerous to handle. If you use a stronger cleaner, work outdoors and wear gloves, eye protection and a respirator as the product label says.

Rinse from the inside (engine side) outward so the dirt washes out the way it came in, not deeper into the grid. A gentle stream of water is enough. Some technicians blow it dry with compressed air from the inside out; if you do, keep the pressure low and the nozzle back from the grid, because a strong blast can bend or separate the fine mesh or plates and ruin the arrestor.

Shake out the water and let the arrestor dry fully before you put it back, which can take a few hours in the sun or overnight indoors. Water or solvent left in the grid will be pulled straight into the engine.

While it dries, wipe the mounting surface on the carburetor or throttle body and check the gasket or seal, if your engine uses one. Replace a cracked or flattened seal; the arrestor must make a flame-tight seal. Remove your rag or tape only when you're ready to fit the arrestor, and check that nothing has fallen in.

Set the arrestor squarely in place, reconnect the crankcase vent hoses exactly as they were, and tighten the fastener firmly without crushing the arrestor or cracking the base. Refit the cover. Make sure no hoses, wires or rags are touching moving parts or the exhaust.

If the arrestor was damaged, will not come clean, or has no approval marking, buy a replacement made for your engine and marked to SAE J1928, UL 1111 or USCG approval. Boating Magazine notes that size and fit matter: measure the carburetor's air inlet flange and check the hatch clearance before ordering an aftermarket arrestor. Never substitute a car-style air filter.

Reconnect the battery, run the blower, then start the engine with cooling water supplied (on the water or on a flush setup your manual approves). Listen for the engine to idle and rev normally. Shut down and check that the arrestor is still tight and that nothing is touching it. If the engine runs better than before, the old one was probably clogged.