Agree with what's been said.
Pull the seat, pull the leads off the battery, pull the two wires connected to the fuel sending unit on top of the large round plate about 12" behind the fuel fill cap, don't be smoking, and tap the tangs of that large round cap with a dull flat screwdriver in a counter-clockwise direction until it's free and pull out the fuel sending unit. Let it dry (with a rag stuffed in the hole in the tank) and check out how crappy it looks. I fixed my '85 simply by spraying it with Gumout after cleaning the curved contact plate with some fine ScotchBrite.
OK, I whacked the "box" (you'll see what I mean) a few times, too, with the plastic handle of a screwdriver! :shock: :Egyptian: :smilie_happy: Has worked just fine ever since, though I never trust a fuel gauge anyway; I know its MPG and go by the odometer.