2 Stroke Outboard Fuel Mix Ratio

50:1 100:1? Finding the right mix for your outboard.

In this guide we step through the why it's vital to get the right 2 stroke fuel oil mix and some methods to work out which ratio to use for your outboard. Finishing with tips on how to mix two stroke efficiently and safely.

Why mix Fuel & Oil anyway?

Any outboard described as a 'two stroke' engine is an internal combustion engine than burins injected fuel through a reduced power cyle of just ignition and exaust (more info about the difference between two stroke and four stroke engines here. ) Due to this design it is necessary to provide the engine with lubrication (the oil) actually within the injected fuel (the petrol or gas). The relative proportions of these two components is called the 'fuel mix ratio'.

Getting the mix right

Every engine will have it's own fuel mix ratio, often mandated by power, intended use and age of the outboard and manufacturers state this required mix ratio and any tolerance (if there is any!) in the owners documentation that came with the original purchase.

Not knowing and running the right mix can cause a whold bunch of problems. Difficulty starting, performance degradation, increased wear & tear of internal components,  excessive smoke from the exhaust to name a few.

Most outboards will tolerate an incorrect mix on occassion but running the wrong fuel oil mix over for any prolonged period of time is likely cause reliability and performance issues at best and potentially irrepairable damage at worst.

Knowing the right mix is simple enough if you're buying a new outboard or were lucky enough to get the original owners manuals when purchasing an secondhand outboard but often these get mislaid as engines passthrough various owners over the course of their lifetime of years or even decades!

If you're in this buck and don't have the original owners manual then you'll need to determine the right mix prior to use.

Here are some suggestions:

Method 1. Search for clues on the outboard - Most reliable!

A good proportion of outboard manufacturers will print/inscribe/stamp the right fuel mix ratio somewhere on the unit itself. This might be really obvious on the cowling like in the image below of a late 1980s Yamaha outboard or it can be a bit trickier to track down being on the underside of the cowling, embossed on the fuel filler cap (outside or inside) or stamped on the side of the fuel tank.

These are the most common locations but if you don't find any reference in these common places look for a reference on any engine number, batch or quality controls stickers or plates.

Method 2. Owners Handbook or Service Manual

If a robust search of the engine doesn't surface anything the next best method is to reference either the original manufacturers owners handbook/operators guide or a service and repair manual. Both manual types will almost always contain a speciation section where key lubricants and recommended fuel oil mix will be stated.

This method does rely on accurate model and year identification which with older outboards might entail locating and looking up the engines serial number.

Looking for a an owners manual? Browse our Library 

Method 3. Download a manufacturers 2 stroke oil mix chart

If there's nothing on the outboard itself and you can't find a manual the next step would be to perform a search of manufacturers websites for reference material and archived docs. Many of the major manufacturers like Yamaha, Evinrude/Johson and Mariner publish 2 stroke fuel oil mix charts in litres and fluid ounces. Many also have online search tools where you can enter your outboards make and model or serial number to get the right mix chart.

Method 4. Perform an online search/forums

If all else fails a quick internet search of your outboard make, model and age will often reveal the required ratio. There a hundreds of sites with rich archives  of data and it's likely that most of the mainstream units are documented somewhere.

If you've got a more unusual outboard or particularly old unit then try posting a query in the one of the boating advice boards or forums. It's likely that somebody somwhere has walked the same path an only to happy to advise!

If all else Fails. The 'Fallback' fuel mix

If all else fails and you're absolutely stuck we recommend a mix of 50:1 for outboards older than 1980 and 100:1 for outboards manufactured since 1980. That is 50 parts fuel to 1 part oil and 100 parts fuel to 1 part oil respectively. By no means perfect but a good general rule.

How to mix 2 stroke fuel

Once you've landed on the ratio you need actually do the mixing.

Always store, handle and mix outboard engine fuel in a well ventilated space, ideally outdoors and away from any source of ignition of chance of contamination, i.e. not on the ground of some dusty car park!

Tip: Sawdust, Sand and even Cat Litter all make effective absorbant materials should you have a fuel or oil spill.

Mixing Checklist:

1. Prepare you fuel, oil on a bench or other suitable surface and have a clean rag and medium sized funnel handy.
2. Add the oil to the fuel rather than the other way around so have a suitable rounded number volume of fuel ready in a container (Starting with 5 Litres of petrol in a tank is ideal)
3. Add the two-stroke oil through the funnel mixing thoroughly.
4. Re-seal both the fuel and oil containers
5. Clean-up an spilt fuel or oil and make sure to commit to memory or note down which fuel tank has the mixed fuel ready to go.

Enjoy your time on the water. Safe boating.

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