1980 Suzuki GS450 transmission problems?

drain tank,get some small nuts and bolts and a gallon of kerosene put that in tank and slosh around to remove all the rust and gunk. Buy product called KREAM which is a three step process and re line the tank with it.[simple to do] Remove the carbs and take apart and clean them. [get a shop manual] re install carbs and synchronize them. If you dont have the tools then send it off to a shop should cost about 300 dollars to do.