Who knows? Rust, varnish/tar, old screen or float parts... Just gotta drop it, scope it, stick a "stick" in the outlet to release the object(s). Maybe run a wire thru the fill hole or sender hole to outlet? Clean, flush, test, install. The usual grunt work we all do and hate. You can use water to test. Hope it's not still under the seat.
This tank is supposed to have two outlets?
