saving file with ASCII encoding

I'm trying to save an R script in Rstudio with ASCII encoding, but get the following error:

"Not all of the characters in filename.R could be encoded using ASCII. To save using a different encoding, choose "File | Save with Encoding..." from the main menu."

The problem is that I've already replaced non-ASCII characters with their right \uxxxx escapes (so for example, I replaced ± with \u00B1), and even functions like showNonASCII() and showNonASCIIfile() do not point to non-ASCII characters.

What can I do next?

thanks