Air filter is fairly new.....still very clean.
One other thought - when I bought the car the rubber pipe from the oil filler/breather cap to the underside of the air filter box was just directed down the back of the engine and the air filter was extremely dirty. After changing the filter I connected the pipe back to the air filter and it shows no sign of excessive fumes etc being drawn into the carb.
Recently the guy I bought the car from saw I had reconnected it and said he thought it was better if not connected but didn't give any reason.