So I'm experiencing this same problem with spaces being added between quotation marks and capital letter Cs and Ks.
I'm converting a docx to mobi with Calibre and then emailing it to my Kindle Paperwhite. The spaces don't show up on my computer previewer, in Word, or even in the overview when I highlight the issue and then go through my "notes" in the Paperwhite.
I've tried deleting and retyping the offending letters, I've tried converting to epub first, I've inspected the "normal" style that they are all under, and I've checked all the Calibre settings I can think of. It's only Capital letter Cs and Ks that aren't italicized so I'm guessing it has something to do with the style, but I can't figure out anything that would only apply to those two letters and only in the Paperwhite.
Some HTML:
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{mso-style-unhide:no;
mso-style-qformat:yes;
mso-style-parent:"";
margin:0in;
margin-bottom:.0001pt;
text-indent:.5in;
line-height:120%;
mso-pagination:none;
mso-layout-grid-align:none;
text-autospace:none;
font-size:12.0pt;
font-family:"Times New Roman","serif";
mso-fareast-font-family:"Times New Roman";
mso-bidi-font-family:"Courier New";}
From an offending sentence:
<p class=MsoNormal>“Come on baby, let’s go,” he was saying.</p>