Printer does not print pure black text. Prints composite CMYK black instead with horrible results

[nofun1]

Hi all. I get nice results printing black text from this printer only when the output is set to "black and white". If I want to print any form of coloured document – say, with a small portion of the text in red, with the rest black – it instead outputs a horrid composite black instead using all CMYK inks. As you can imagine, this leads to a lot of ink spread, and the result is a disappointing blue/grey rather than the nice black black that the "black and white" setting outputs.

I've checked my file thoroughly and can confirm that the text itself is set to 100% K, not a rich composite. I've also adjusted every setting I can find in InDesign and Acrobat. The printer continues to disregard CMYK colour mix and applies its own, hence turning the pure 100% K to a hideous composite.

Is there any way to fix this? As the printer is printing fine when forced to black and white there is clearly no mechanical issue with the printer. I just need confirmation that this is an unavoidable feature of this printer before I discard it. My main requirement is to print black text with limited areas of colour, so there is really no advantage at all in my text being printed with a composite 'black' especially when it is less black than pure 100% K.