plugged air correction jets? RUST

[chiroracer]

I have been sorting out and "fine tuning " my 2.7, rs pistoned, solex cammed , weber carburetted engine. It just keeps running better and better as I find and correct small things at the suggestion of many fellow pelicans.I was noticing a lean pop and surge at 3000 rpm and was installing some smaller air correction jets 160 from 180 and I discovered that the left bank of ac jets were plugged with black chunks,I deduced that these must be from my insulation pad when I had the air cleaner off ,I cleaned them and re installed them and the emulsion tubes, f3,
I had a magnet out in case I dropped a nut and noticed that these chunks were magnetic.
Where from??? I looked inside my stock airbox bases and they are virginous with original paint and patina,I looked again and was shocked ,waaay up under the lip where the rubber gasket lives there was a very small(visually) rust spot but when I poked it with my screwdriver it gave way with a sprinkle of the offending particles. My point is this was almost invisible but had all but totally blocked the orofices on the left side. Pull the rubber gasket all the way out of the trench and check yours. Steve