There is a product called "Rusteco" that I have used on several occasions to clean out rust on the inside of a gas tank. It is a citrus acid based product, somthine along the lines of Simple Green or the Orange degreasers, but apparently much stronger. I have cleaned out the tanks on each old bike I have purchased, and after being done, they are clean as a whistle. Great thing is, while it reacts with the rust, it does not eat or etch the metal, like the strong chemical de-rusters I have used (such as Nautical-Ease).
I recently had to rebuild a bank of 4 carbs on my CB750 and I thought I would dip them in this product since they had about 30 years of fuel, grime and corrosion on them. I was not able to clean this stuff off fully even with Brake cleaner, and I was afraid the brake cleaner would eat the rubber and nylon parts of these carbs.
I dipped the entire bank in Rusteco for 6 days. Wish I had taken before and after pics, because the carbs looked like new when I took them out of the bath. The bell crank and frame were completely covered in rust, as where just about ever steel part on the carbs. Rust, corrosion, grime all gone, whether the underlying part was steel, aluminum, nylon, rubber or plastic. There was no etching of any of the metal parts, just needed to rinse in water, blast with air, allow to dry, and reassemble.
The instructions with the Rusteco state for a rusted gas tank to allow the product to soak in for 2 days, rinse, then soak another day. I find letting it soak for 6 days straight is better, and since it does not etch, there is no problems of "overdoing" things...
Carb Cleaner
Carb Cleaner
1982 Laverda 180 JOTA, 1975 Moto Morini 3 1/2 Sport, 1993 Moto Guzzi Daytona, 2010 KTM Adventure 990, 1974 Laverda SFC #17188