Rust or flakes inside the fuel tank

Rust forms inside steel tanks when water sits in them, usually from condensation in a tank left part empty, or from a leaking cap. The flakes break off and travel with the fuel. They block the tap strainer and filter, wedge the carburettor float needle open (overflowing) or shut (no fuel), and clog the pump strainer on injected bikes. Cleaning the filter helps only until more rust arrives. Bad rust can also eat through the tank and make it leak.

Is it safe to ride?

With care, after fitting a clean filter, while you arrange a proper fix. If the tank is leaking or wet on the outside, do not ride: see Fuel is leaking or dripping.

Quick checks first

• Is the inline filter or tap strainer orange or brown?
• Look into the tank through the filler with a bright torch: rust on the walls or bottom?
• Is the outside of the tank wet, bubbled or stained near the bottom or seams?
• Has the bike stood with a half-empty tank, or does the cap seal leak?

Diagnose it step by step

1Shine a torch into the tank. Do you see rust?

• Light orange dust on the walls: early rust. Go to step 2.
• Brown flakes, scale on the bottom, rust around the tap or pump hole: active rust. Go to step 3.
• None, but rust in the filter: rust may come from a steel fuel tap or line. Go to step 2.

2Drain a fuel sample into a clear jar. Are there flakes or water?

• Yes: Drain the tank, rinse with a little fresh fuel, and fit a new filter: Replace the fuel filter. Clean the tap strainer: Check the fuel tap, lines and flow. Watch the new filter for a few tanks.
• Filter stays clean: done. Keep the tank fuller and check the cap seal: Clean and lubricate the fuel cap.
• Filter keeps filling with rust: go to step 3.

3Is the outside of the tank sound (no pinholes, bubbles or wet seams)?

• Yes: the tank can be treated. Workshop: Remove rust from a fuel tank.
• No: the tank is weak or holed. Drain it and replace it, or have a specialist repair it. The K-Light 125 manual says a rusted or leaking tank is replaced. Never weld, braze or grind a fuel tank unless a specialist has made it vapour-free.

4After treatment, check what the rust has already reached.

• Carburettor: flakes in the bowl or on the float needle: Clean the carburettor.
• Fuel-injected: the in-tank pump strainer should be cleaned or replaced while the tank is out: Check the fuel pump.

Causes, most likely first