Hyundai Replacement Sponge Main Air Filter for Petrol Mowers This Replacement Sponge Main Air Filter is suitable for use with the HYM400P Hyundai Petrol Lawn Mowers and P4100 P1 Power Equipment Petrol Lawn Mower. Air filters on a petrol lawn mower prevent any dust and other particles from entering your engine. You should aim to replace this filter approximately every 3 months (depending on use), or whenever it is excessively dirty, for lawn mower longevity.