Internal combustion engines can backfire. In order to prevent flames from the backfire causing a fire onboard, an approved Backfire Flame Arrestor must be attached to the air intake with a flame tight connection. Flame arrestors must be in good and serviceable condition and approved by the U.S. Coast Guard, Underwriters Laboratories (UL 1111) or the Society of Automotive Engineers Standard (SAE - J-1928). All motorboats have this requirement except outboards and diesels.
It is very important to insure that your Flame Arrestor is clean and undamaged. If there is a hole in the grid, or oil or gasoline in the grid, or if it is not properly attached, it will not work correctly. To clean the flame arrestor, remove it and wash the heat dissipation grid with soap and water. Let dry thoroughly and reinstall. Make sure that the arrestor has a flame-tight connection.