I assume the fuel gauge works properly and indicates the correct fuel level when the lights are switched off. I also assume that the headlights, tail lights, and the two dash lights are working. If those assumptions are incorrect then any of those issues should be fixed first. The fuel level gauge varies ground continuity for the signal to the fuel level gauge. It's never a bad idea to check and clean the contacting point of the frame grounds. If you have a multimeter, check the voltage at the Red/Yellow wire on the back of the fuel level gauge and confirming it shows battery voltage. With the key off, check the resistance between the Black wire back to the negative post of the battery to confirm good continuity. The Yellow/Blue wire should indicate the resistance through the fuel level sensor. If I were working on the machine I would want to see what, if any, difference it made when the key switch was just turned to the ACC position versus having the engine running. The second thing I would want to see is the effect it would have if the regulator was disconnected. There are some additional things that could be checked but I would start with those. Let us know what you find.