It's on the side of the block under the intake manifold. Provides a place for crankcase oil vapor to condense and drip back into the pan. If the oil has been changed on schedule, there shouldn't be any need to clean it but if it is plugged, it will run the CC pressure up and probably blow a seal. Haven't done it my 850 yet but on my old 740 I'd remove the flame trap once a year and insert a funnel into the large hose and dump a can of GUNK Motor flush ( or kerosene) in. Has to be done during an oil change with the oil and drain plug out. The GUNK should run right through the trap and out the drain hole. If it doesn't or trickles through, the box is plugged and the GUNK will probably do magic on it. If it doesn't, the trap has to be removed for cleaning which involves removal of the intake manifold etc.