Other posters are correct in doing a tune up first. Here's another thought. That gauge likely gets it's reading off of vacuum. An old trick my dad showed me many years ago to find vacuum leaks was to spray WD40 on the vacuum hoses. The engine would idle kind of rough if there was vacuum leak somewhere. The challenge was finding the vacuum leak. If the WD40 was sprayed on the leaking portion of the vacuum hose it would seal it for a moment and the engine would smooth out for that moment. Eventually I would figure out where the hose was leaking and fix it. It was usually at the end of the hose that was on a nipple, and cutting off a little bit would would give it a tighter fit and eliminate the leak. Hope that's helpful.
Harley Ultra Classic (Geezer Glide) Rider, Retired US Army Paratrooper, fisherman, shooter. Proud to have served, proud of those that still do, or have done so with pride.
2005 National Dolphin 34'