[Hugh R]

Sand/grit through the air cleaner. In my old Aston Martin DB4 I had SU carbs with a wire mesh for an air filter.The wire mesh wouldn't stop anything from entering the engine. I sandwiched some oil-soaked motorcycle foam air filter material between the wire mesh so it looked original, but actually worked, and wouldn't suck the foam into the SUs. Had to adjust the carbs for the change in air flow, IIRC.