If you do a seacrc under my user name you should find an post I did about CB antennas.
Replace the cable, the RG58 mini sould be a good cable in the HD. Now remember a MC has basically no ground plane effect so it is a little difficult to get a good SWR. I went all the way & installed a complete non ground plane Firestick. It worked much better but looked ugly with the 3' Firestick antenna.
I went to the short antenna's sold by Harley, however they are not adjustable to get the SWR correct. I started off with a 17' RG58 cable as that was the correct cable length for non ground plane, same cable I used with the Firestick. I kept shortening it & rechecking the SWR until I got it in an acceptable range.
With the Firestick I was less than 1.5:1 all through the 40 channels. With the short antenna I am as high as 1.7:1, still ok. I did tune it to best fit the channels I use, 23 to 27 and in that range I am 1.2:1. 17 & 19 are about 1.4:1
This is a PITA process, cutting cable & reinstalling the connector, then testing SWR. I used the twist on Radio Shack for all the test then installed a good solder connector.
Remember all SWR testing should be out in the open, you can't do it in your garage & get it right. Try and have at least a clear 15' circle all the way around the MC.
The connector at the back of the CB is a TNC, not standard for a CB. You will also need to make an adapter cable & have a couple adapters to use and SWR meter because of the TNC connector. The Tourpack Connector is crap, get rid of it, it is an automotive radio antenna connector, cheap, easy disconnect but totally wrong for CB. Just run the CB cable all the way to the antenna.
Hope this helps.