I think ethanol in fuel has a lot to answer for. Its surprisingly reactive and seems to attack quite a lot of materials and absorbing water just adds another element. When they upped the ethanol in diesel (which they keep quiet about) my Land Rover injection pump turned into a sieve with leaks everywhere.
On my A10 the Concentric pilot circuit was solid and nothing would budge it. I drilled the fixed jet out, tapped the carb body and fitted the earlier threaded jet so I can now poke it though.