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Petrol for Outboard Engines: What Every Operator Needs to Know

Petrol is the lifeblood of an outboard engine — but not all fuel is created equal. Whether you’re running a 2-stroke or 4-stroke, using the wrong petrol (or storing it poorly) can wreck an engine faster than a submerged log.

This article breaks down the essentials: the types of petrol used in marine outboards, the risks of modern fuel blends (especially ethanol), and how to handle and store it safely.

1. The Basics: What Kind of Petrol Do Outboards Use?

Most outboard engines — both 2-stroke and 4-stroke — run on unleaded petrol. But which grade?

  • Regular Unleaded (91 RON/87 AKI) is suitable for most small to mid-range outboards.

  • Premium Unleaded (95–98 RON) may be recommended for high-compression, performance engines.

Always follow the manufacturer’s spec. Using lower-grade fuel than recommended can cause knocking (pre-ignition), which severely damages engines.

Training Tip: Make new owners check the cowling decal or owner’s manual — don’t assume “all petrol is the same.”

2. Ethanol-Blended Petrol: A Marine Headache

E10 petrol was introduced as the new standard grade of unleaded petrol in Great Britain in September 2022 and in Northern Ireland in November 2022. It replaced E5 as the stand unleaded petrol available at the pumps

What is E10?

E10 fuel contains 10% ethanol — a form of alcohol. While it’s legal and widely sold at service stations, it’s not ideal for marine use. It is however part of the UK’s drive towards less CO2 emissions.

Why Ethanol is a Problem for Boats:

  • Attracts Water: Ethanol is hygroscopic — it absorbs moisture from the air, leading to phase separation (water and fuel separating inside the tank).

  • Corrosive: Ethanol degrades rubber hoses, gaskets, and older fuel system components.

  • Shelf Life: Goes stale faster — often in as little as 3 months. This can be a huge problem if your marina fuel berth has a slow turn over of fuel as it may have already sat in their tank for 1-2 months before you buy it, especially in the off season and remote locations

  • Reduced Power: Ethanol-blended fuel has slightly less energy content than pure petrol.

Bottom Line: Avoid E10 if possible. Use ethanol-free marine-grade fuel when available.

3. Fuel Storage: Do It Right or Pay Later

Problems with Poor Fuel Storage

  • Fuel oxidizes and breaks down. (Over time, especially in the presence of air, fuel degrades through oxidation. This leads to the formation of acids and sticky compounds that reduce combustibility and can corrode fuel system components. This is particularly an issue with Petrol (gasoline) and biodiesel.

  • Water contamination from condensation. As temperatures fluctuate, condensation can form inside fuel tanks—especially if they’re not kept full. Water sinks to the bottom of the tank (it’s heavier than fuel). It can lead to rust, microbial growth, and poor combustion.

  • Gum buildup in carbs and injectors. As fuel degrades, it forms sticky residues. Gum clogs carburettor jets and floats. It can also stick injectors and block fuel passages..It leads to rough running, poor starting, and engine damage

Best Practices

  • Use stabilizer: Always treat stored fuel with a marine fuel stabiliser, especially if it is E10

  • Keep tanks full: Minimizes air (and moisture) in the tank.

  • Rotate stock: Don’t use petrol older than 3 months without stabilizer, or 6–12 months with stabilizer.

4. Mixing Petrol for 2-Stroke Outboards

For 2-stroke engines without oil injection: Forgetting to add oil can lead to piston seizure, Adding too much oil can lead to fouled plugs.

  • Correct fuel-oil ratio is critical. Common ratios:

    • 50:1 (most modern 2 stroke outboards)

    • 100:1 (some Evinrude E-TECs)

    • 25:1 or richer (older 2 stroke models)

  • Use marine-grade 2-stroke oil, not chainsaw or lawnmower oil.

Mixing example for 50:1: 1 litre of oil for every 50 litres of petrol (or 100 ml per 5 l).

5. Fuel System Maintenance

  • Inspect fuel lines regularly for cracks, softness, or leaks.

  • Use a proper fuel filter/water separator — especially in high-moisture environments or if using E10

  • Drain carbs or run engine dry before long storage.

  • Replace stale fuel rather than risk clogged injectors or gummed jets.

6.. Summary: The Golden Rules of Outboard Petrol

Rule Why It Matters
Use the right octane Prevents knocking and pre-ignition
Avoid ethanol if possible Water issues, corrosion, poor shelf life
Store fuel correctly Prevents gumming, condensation and fuel system failures
Mix oil properly (2-stroke) Lubrication is life
Use fresh fuel Old fuel = performance and starting issues

Final Thoughts

Fueling outboards isn’t just about “filling it up.” It’s about knowing what’s in your tank, how long it’s been there, and how it interacts with your engine. With ethanol-blended fuels now widespread, good fuel management is critical for engine health — especially in training and operational fleets where reliability is everything.