Running Legacy 2-Stroke Outboards on Modern Ethanol Fuel

Best Fuel for Old 2-Stroke Outboards: Ethanol vs Recreational Gas for Mercury, Johnson, Evinrude, Yamaha, OMC!

If you own a legacy 2-stroke outboard and you’re filling up at today’s gas pumps, you’re running fuel that your engine was never originally designed to see. Modern gasoline with ethanol, typically E10 (up to 10% ethanol), behaves very differently than the straight gasoline that powered classic Mercury, Johnson, Evinrude, Yamaha, and Tohatsu 2-stroke outboards for decades.

For performance-minded owners and racers, understanding the difference between ethanol pump gas and recreational ethanol-free fuel is critical. It affects tuning, reliability, storage, and ultimately how long your engine lives.

This technical guide breaks down what changed, what it means for your legacy 2-stroke, and how to fuel and set up your outboard for today’s gasoline.

From Old-School Gas to Modern Ethanol Blends

When older 2-stroke outboards were designed, they ran on conventional gasoline with no ethanol and different additive packages. Today, most roadside fuel is reformulated gasoline with ethanol added as an oxygenate to reduce emissions.

Key differences between old fuel and modern ethanol blends:

• Ethanol content – Most pump gas is E10 (up to 10% ethanol by volume). Ethanol carries oxygen and has less energy per gallon than pure gasoline.

• Energy content – Ethanol-blended fuel has slightly lower BTU content, which can translate into a leaner effective mixture and a small drop in power and fuel economy.

• Hygroscopic behavior – Ethanol attracts water from humid air and can lead to phase separation in storage.

• Solvent action – Ethanol can loosen varnish and deposits in old fuel systems, and can be harder on certain older rubbers and plastics.

Your legacy 2-stroke outboard will usually run on E10, but ignoring these differences is how you end up with clogged carbs, water-contaminated fuel, or a lean-burned piston.

How Ethanol Affects Legacy 2-Stroke Outboards

Whether you run a Mercury 2.5L, a Johnson / Evinrude looper, a Yamaha V4/V6, or a Tohatsu 2-stroke, the fundamentals are the same. Here’s what modern ethanol fuel does to older two-stroke outboards.

1. Leaner Effective Mixture

Ethanol contains oxygen and less energy per unit volume than pure gasoline. When you pour E10 into a carbureted 2-stroke that was jetted for old fuel, the carburetor doesn’t know the difference – it meters roughly the same volume. But because the fuel has less energy and contains oxygen, the engine sees a leaner effective air–fuel ratio.

For stock or lightly modified fishing engines that are correctly jetted and timed, this is usually safe. For high-compression, high-RPM, or performance-ported engines, that leaner mixture cuts into your margin of safety and increases the risk of:

• Detonation or “pinging” under load

• Elevated exhaust gas and piston temperatures

• Scuffed pistons and ring land damage

If your 2-stroke is set up on the ragged edge for max power, common Pump Fuel (E10) is not “just gas” – it’s a tuning factor.

2. Slight Power and Economy Loss

Because ethanol has less energy than pure gasoline, most engines see:

• A slight drop in top-end power

• A small reduction in fuel economy

On a stock fishing outboard this may be barely noticeable. On a dialed-in performance motor, you may see a difference in RPM and throttle response.

3. Fuel System Clean-Out and Contamination

Ethanol acts as a solvent and can loosen old varnish and deposits inside tanks, lines, and carburetors. This is especially true when a boat that sat with old fuel is suddenly fed fresh E10. The result can be:

• Clogged filters and water separators

• Restricted carb jets and idle circuits

• Debris in needle seats and inlet screens

When transitioning an older outboard to ethanol fuel, it is common to need more frequent filter changes and sometimes a full carburetor cleaning or rebuild as the system “cleans itself out.”

4. Compatibility With Older Rubber and Plastic

Many engines and boats built after roughly the early 1980s use fuel system components that tolerate ethanol well. Older hoses, bulbs, and seals, however, may soften, crack, or weep.

For any legacy Mercury, OMC (Johnson / Evinrude), Yamaha, or Tohatsu outboard that will see E10, you should:

• Replace old gray or soft fuel line with ethanol-rated USCG-approved hose

• Install a fresh, good-quality primer bulb designed for ethanol-blended fuel

• Inspect all under-cowl fuel hoses, grommets, and seals and replace anything hardened or cracked

  
  

This is cheap insurance compared to a lean failure caused by an air leak or a collapsed hose.

5. Water, Phase Separation, and Storage

Ethanol is hygroscopic – it absorbs water from the air. Over time in a vented boat tank, moisture can accumulate. When water content climbs high enough, ethanol and water can separate out of the gasoline and settle at the bottom of the tank as a heavy layer.

That layer is what your fuel pickup will grab first when you start the engine after storage. Consequences can include:

• Hard starting and rough running

• Corrosion inside carbs, pumps, and injectors

• Severe lean conditions if the main gasoline layer is depleted

  
  

This is one of the biggest drawbacks of E10 in seasonal, rarely used, or stored boats and a major reason many owners move to recreational ethanol-free marine fuel whenever possible.

Fuel Choices: E10 vs Recreational Ethanol-Free Fuel

For most two-stroke outboard owners today, the realistic options are:

• E10 pump gas (up to 10% ethanol)

• Recreational ethanol-free marine fuel (often labeled REC-89, REC-90, REC-93 non-ethanol, or boat fuel)

  
  

(E10) Pump Gas - Often 87, 89, 91, 93 Octane

E10 is the default at most roadside gas stations. For a modernized, well-maintained fuel system and a stock or mildly modified engine, it can work reliably if you:

• Use fresh fuel and avoid long storage with E10

• Upgrade to ethanol-rated hose and components

• Install and regularly service a 10-micron water-separating fuel filter

• Ensure carburetor jetting and ignition timing are correct for the load and environment

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For many everyday fishing outboards, E10 is acceptable as long as you pay attention to maintenance.

Recreational / Ethanol-Free Fuel (i.e. REC-90)

Recreational gasoline or “marine fuel” is ethanol-free and much closer to what older outboards were designed to run on. Benefits include:

• No ethanol, so no phase separation and reduced water absorption issues

• Higher energy content, which can improve throttle response and fuel economy

• A slightly richer effective mixture, which is gentler on high-output 2-strokes

• Less aggressive behavior toward older hoses and seals

  
  

For high-performance Mercury 2.5L racing builds, hot Johnson/Evinrude loopers, performance Yamaha outboards, and any legacy 2-stroke that lives at high RPM and heavy load, ethanol-free fuel is strongly preferred. It is also ideal for boats that sit between outings or are stored seasonally.

A smart strategy for many owners is to run E10 when necessary but switch to ethanol-free fuel for the last few outings of the season, stabilize that fuel, and store the boat with an ethanol-free mixture in the tank and system.

Tuning Tips for Running E10 in Legacy 2-Strokes

If you must or choose to run E10 in older Mercury, Johnson, Evinrude, Yamaha, or Tohatsu 2-stroke outboards, take the time to set the engine up correctly.

Modernize the Fuel System

Replace any questionable components in the fuel path:

• Tank pickup and lines

• Primer bulb

• Under-cowl fuel hoses

• Quick-connect fittings

  
  

Use ethanol-compatible marine hose and quality clamps. Air leaks in the suction side of the system can lean out the engine and cause surging at high RPM.

Add a Water-Separating Fuel Filter (Unless you Drain Your Fuel Regularly)

Legacy 2-stroke running E10 should have a 10-micron water-separating fuel filter between the tank and the engine. This filter captures both water and loosened contaminants from the tank, protecting carburetors or injectors.

Mount it where it is easy to inspect and change. For boats transitioning from old fuel to E10, plan on early filter changes until the system stabilizes.

Check Jetting, Mixture, and Sync

On carbureted engines:

• Verify that carb synchronization is correct

• Confirm idle mixture and transition circuits are clean and properly adjusted

• Consider slightly richer main jetting on heavily loaded or performance applications when using E10, especially in hot weather and heavy boats

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A plug check at your typical cruise and WOT settings is invaluable. On high-performance engines, many builders choose to tune assuming ethanol fuel so that a surprise tank of E10 doesn’t push the engine over the edge.

Verify Ignition Timing and Cooling

Running slightly leaner on E10 means the engine has less thermal margin.

Make sure:

• Ignition timing is set to factory specification or slightly conservative for performance builds

• The cooling system is healthy: fresh water pump impeller, clear passages, working thermostats or poppet valves

Do not push timing or compression to the edge and then add ethanol on top. Build in margin.

Use Quality Oil at the Correct Ratio

Two-stroke oil is part of your fuel system.

• Use a premium TC-W3 outboard oil

• Mix at the ratio recommended by the manufacturer or set your oil injection system correctly

• Resist the urge to run less oil “because modern fuel and oil are better” – that logic, plus leaner fuel from ethanol, is a fast way to damage a high-output 2-stroke

Storage Strategy for Legacy 2-Stroke Outboards

How you store your fuel matters just as much as what fuel you choose.

For boats running on common Pump Gas (E10):

• Avoid letting E10 sit in vented tanks for long periods

• Either run the tank down and drain carburetors, or stabilize a full tank of known-fresh fuel and run the engine long enough to pull treated fuel through the system

• Store in a cool, dry environment wherever possible to reduce moisture absorption

For boats using ethanol-free recreational fuel, storage is more forgiving, especially when combined with a good marine fuel stabilizer. Many owners deliberately switch to ethanol-free REC-90 for the last stretch of the season and store the boat with ethanol-free fuel in the tank and carbs.

Regardless of fuel type, always:

• Run the engine on the hose or in the water after winterization or layup work to confirm proper operation

• Check the water-separating filter at the start of each season and replace it if there is any sign of water or sludge

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Brand-Specific Considerations (Mercury, OMC, Yamaha, Tohatsu)

While each manufacturer publishes its own specifications and guidance, the practical rules for running legacy 2-strokes on modern fuel are very similar across brands:

• Mercury – Many classic 2.0L, 2.4L, and 2.5L carbureted V6 engines can operate on E10 if correctly jetted, with a clean, ethanol-capable fuel system and proper filtration. Performance builds benefit from ethanol-free premium or racing fuel and careful jetting.

  
  

• OMC / Johnson / Evinrude – Loop-charged and cross-flow engines will run on E10 but can be sensitive to lean conditions, dirty carbs, and weak fuel pumps. Ethanol-free gas and rich, safe jetting are especially important for high-RPM or ported engines.

  
  

• Yamaha – Many Yamaha 2-stroke outboards tolerate E10 well when maintained, but their precision carburetion and, on some models, oil injection systems make clean fuel and a healthy fuel system crucial.

  
  

• Tohatsu – Smaller two-stroke Tohatsu engines are robust, but like the others, they benefit from ethanol-rated fuel components, proper filtration, and preference for ethanol-free marine gasoline where available.

  
  

In all cases, fuel quality, filtration, and correct tuning matter more today than they did when these engines were new.

Key Takeaways for Legacy 2-Stroke Fueling

If you want your older 2-stroke outboard to survive and perform on modern gasoline:

• Recognize that E10 is different: slightly leaner, lower energy, and more sensitive to storage.

• Modernize your fuel system with ethanol-rated components and a quality water-separating filter.

• Take the time to tune carburetion and timing for the fuel you’re actually running.

• For high-output, high-RPM, or racing applications, strongly consider recreational ethanol-free marine fuel with proper octane.

• Treat long-term storage as a fueling decision, not just a fogging and anti-freeze job.

Legacy Mercury, Johnson, Evinrude, Yamaha, and Tohatsu 2-stroke outboards can run safely on today’s fuels – but they will not tolerate being treated like they’re living in 1985. Understand the fuel, set the system up correctly, and your 2-stroke will keep pulling hard, season after season.

*All Octane Numbers in this article are based on the common AKI Rating used in the USA.