Steel tanks rust inside when water sits in them: from condensation in a half-empty tank, a leaking cap, or dirty fuel. Rust flakes block the tap strainer, fuel filter, carburettor jets and the pump strainer on injected bikes. Cleaning the filter only helps for a while; the rust keeps coming. This page tells you how to judge the tank and what the workshop does. Treating a tank uses strong chemicals and works with fuel vapour, so it is workshop work.
Safety
- An "empty" fuel tank is full of explosive vapour. Never weld, braze, grind or drill a fuel tank, and never use heat on it, unless a specialist has made it vapour-free.
- Rust removers and tank liners are corrosive or toxic. Use them only with the maker's instructions, gloves, eye protection and good ventilation.
What you can check safely
- Park the bike outdoors, engine cold. Remove the fuel cap.
- Shine a bright torch into the tank. Use a phone camera through the filler to see the bottom and the seams. - Light orange dust on the upper walls: early surface rust. - Brown flakes, scale on the bottom, or rust around the tap or pump opening: active rust that will keep blocking filters. - Pinholes or a wet seam outside: the tank is leaking. Stop using it. The K-Light 125 manual says a rusted or leaking tank is replaced.
- Drain some fuel into a clear jar. Orange or brown flakes or water confirm it. See Drain old or contaminated fuel.
- Look at the fuel filter and tap strainer for rust: Replace the fuel filter.
What the workshop will do
- Remove the tank, take off the tap or pump, the fuel sender and the cap, and drain it fully.
- Loosen heavy scale mechanically, for example by shaking with a suitable loose medium, then flush.
- Treat the inside with a rust remover made for fuel tanks, rinse, and dry it fully so it cannot flash-rust.
- Coat the inside with a fuel-resistant tank liner if the metal is thin or pitted and the liner suits the fuel you use. A poor liner can peel and block the fuel system worse than rust.
- Replace the tank if it is perforated or the seams are weak.
- Fit a new fuel filter and new tap or pump seals, and refit with fresh fuel.
What to ask for
- "Please clean and dry the inside of this tank, tell me if the metal is thin or holed, and fit a new filter. Ask me before applying a liner."
Preventing it
- Keep the tank full when parking for more than a few days, so there is less air to condense water.
- Check the cap seal: Clean and lubricate the fuel cap.
- Buy fuel from busy stations and avoid filling from open containers.
Check your work
- After treatment, fuel in a clear jar from the tap or filler stays clean, and the new filter stays clean after several tanks.