Signs of Bad Fuel (and Why It Goes Bad)
Bad fuel is one of the most common reasons small engines like strimmers, chainsaws, hedge trimmers, and mowers become hard to start or run poorly. Two-stroke engines are especially sensitive because fuel is mixed with oil and stored in small tanks.
Signs Your Fuel Has Gone Bad
1. Hard starting or no start
The engine cranks but won’t fire properly, even when everything else is correct.
2. Rough running or stalling
It may start but:
- Dies quickly at idle
- Hesitates under throttle
- Runs unevenly or “bogs down”
3. Loss of power
The machine feels weak, especially under load, even though it usually performs well.
4. Sour or stale smell
Old petrol often smells “varnishy”, sharp, or sour compared to fresh fuel.
5. Visible separation or cloudiness
In two-stroke mix, you might notice:
- Layers forming in the fuel can
- Cloudy or dirty-looking mixture
- Small particles or gum-like residue
6. Carburettor problems
Bad fuel can lead to:
- Blocked jets
- Sticky diaphragms
- Fuel starvation issues
Why Fuel Goes Bad
1. Ethanol absorption (moisture contamination)
Modern petrol often contains ethanol, which absorbs water from the air. This leads to:
- Water contamination
- Poor combustion
- Internal corrosion in the fuel system
2. Oxidation (air exposure)
When fuel sits in a can or tank, it reacts with oxygen and slowly breaks down, forming gums and varnish that clog small engine parts.
3. Fuel separation in two-stroke mix
Oil and petrol can separate over time, meaning the engine no longer gets the correct lubrication ratio.
4. Short shelf life of modern petrol
Even in sealed containers, petrol starts degrading in as little as:
- 30–60 days (regular pump fuel)
- Faster in warm or humid conditions
How to Prevent Fuel Problems
- Use fresh fuel regularly (don’t store it for months)
- Add fuel stabiliser if storing equipment long-term
- Shake two-stroke mix before each use
- Drain fuel if storing machines for winter
- Store fuel in sealed, approved containers
Pro Tip
If your machine suddenly becomes difficult to start after sitting unused for a few weeks, fuel is usually the first thing to check before anything else.
