Kohler: Slow Flushing Toilet

[Colt Hero]

Toilet is flushing very slowly - has been for years. On this trip it was leaking thru the ceiling to the bottom floor, so after ripping the ceiling open below, I replaced the wax ring and that looks OK now, but the toilet was still flushing about the same. Next thing done was to fish a metal hanger through all the holes under the rim. This seemed to help a bit - a little more forceful flush. Then I removed a styro insulation kit I had put in as a boy and scraped and scrubbed inside the tank. The styro kit was installed way back because the tank would sweat very badly in the winter (still does). The windows also sweat very badly in the winter (you can wring a half gallon off each one ... these problems have never been solved despite several attempts). The tank is very clean now, but still a slow flush. Downstairs toilet directly below uses same soil pipe and it flushes fine. No obstruction found in vertical pipe between them.

I'm considering chemicals now. Muriatic acid possibly. Can I use this without damaging anything. Pipes are cast iron, about 70+ years old, but appear to be in decent shape (on the outside, at least).