Not sure about your 750, but my VMax has a sensor in the tank, set so a small filament is placed corresponding to the level of about .9 gallons left. At all times, current is passed through the filament, but the gasoline acts as a heat sink and prevents it from heating up. When the filament is cool, it has a very high resistance. When the fuel level drops below the filament, it begins to heat up. As it warms, it's resistance drops and power is slowly allowed through...why the fuel light slowly fades on, and never gets very bright. This also trips a relay that shuts off power to the fuel pump...this is Yamaha's way of simulating an actual reserve valve, and to ensure the bike "gets your attention" should you miss the fuel light(the bike will start to spit and sputter a minute or two after you see the light). Flipping the switch to "res" simply restores power to the fuel pump and allows the rest of the tank to be used. It should be noted to always leave this in the "on" position except when needed. Otherwise, you will get no noticeable warning(other than the dim fuel light, which is very easy to miss in bright daylight) until you are completely out of fuel.I found out the hard way, that my fuel light doesn't work anymore, and my tank is always on reserve. This is on my '94 YZF750. I check the bulb, and it works fine.
So what can cause such a scenario? Are they related, or just coincidence?