Fuel Sender Unit. Several hours yesterday, same for today. Removed, disassembled, cleaned, adjusted the wiper arms and reinstalled. Still no fuel warning light. Yes, I pulled the instrument cluster, pulled and checked resistance on the bulb and it is good. The only thing I haven't been able to figure out is how to test continuity of the wiring from the plug at the back of the gas tank to wherever those 4 wires go to the front. The Clymer repair manual shows the wiring for a K75RT which has a gas gauge in addition to low fuel light. K75 only has low fuel light so the wiring schematic was hard for me to follow. I had an "early on" light and adjusted the gear wheel on the underside of the sending unit. Then, I lost the light altogether. I have decided that I'll forget about the fuel warning light and rely on the odo, as I know about how many miles I can go before sweat pops out on my forward and I am in panic mode looking for a 100% gas station.
I followed the excellent write up done by poster robmack back in 2014 on the K100 forum as my guide for bench testing.