Fuel Gauge Help

[CEPAwings]

New here and recently acquired an 86 gl1200 Interstate and I’m having a pretty mild issue with the fuel gauge. I’ve noticed it goes from full to empty after around 100 miles. Cooling gauge works fine. Any ideas what would cause it? I was thinking maybe a float but if it works at full I wonder why it’d pick a random time to go to empty. Any help would be greatly appreciated.