For a 3.5 gallon tank, even at $4/gallon, you'll be out $14 for a full tank of fresh gas. I'd drain the carbs and inspect the fuel for bad color or sludge and dirt. Then put in 3-4 ounces of SeaFoam in the tank and top it off with fresh gas. Run that tank as close to empty as you dare, and repeat the process with the second tank.
This should un-varnish the fuel system and carbs, and will help loosen up some carbon deposits on the piston tops. I've seen some YouTube videos of people flooding their cars with SeaFoam just to make big smoke. Back in the day, we used to drizzle transmission fluid down the carb throat when the engine was on high idle. It worked well to decarbonize the heads and combustion chambers. I am not suggesting you do this with your bike. The SeaFoam treatment, done in moderation, is a better way to go.
Re: the carb issue. You can try tapping the carb bowl with a wooden hammer handle and see if it frees up. If it does, then you're probably OK, and the SeaFoam cycles will free it up more. If not, you may have to pull the carbs and tear them apart and clean 'em.
Luke N
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.