I have a 1999 Road King. At some point before it became mine, the fuel injection was replaced with an S&S carb. It came to me with RA8HC Champion spark plugs. I have been researching what plugs to use. This seems to be the correct plugs, but their standard gap is .028. On the Harley side, everything I have read says to use plugs gapped at .040. You can open the RA8HC plugs up to .040, but it is obviously not the gap these plugs are supposed to have.

The bike sat up for a while before I bought it. It would occasionally backfire after I bought it, and the RA8HC plugs gapped at .040 seems to have improved it noticeably and it has not backfired. I made sure the gas was good and added a little sea foam. The old plugs were black and I'm not sure if it was from rich starts and not riding it enough or the carb is not jetted properly.

My main thing right now is the correct plugs. Which plugs do you say to use and what gap?

 

.38-.42 is the spec for the plug gap. the hd 6R12 is oem. ngk dcpre7 works well. if your bike runs well with the plugs you have at the correct gap leave it be till its time for new ones.
m
i'm assuming your bike is a TC88 from the plug number that you posted