What is the fuel mix ratio for a 2-stroke outboard?

Most two-stroke outboards built from the mid-1960s onward run 50:1, which is 2.56 fl oz of TC-W3 oil per US gallon of gasoline, or 20 ml per litre. Motors older than that are usually richer, commonly 24:1 or 25:1 on pre-1964 Johnson and Evinrude units, and a new or freshly rebuilt powerhead is often broken in at 25:1 for the first few hours. The decal on the cowl or the ratio in the owner's manual always wins over a general rule.

How much oil do I put in a 6 gallon outboard tank?

At 50:1 a 6 US gallon tank takes 15.4 fl oz of oil, which is a little under half of a 32 oz quart. At 25:1 the same tank takes 30.7 fl oz, so nearly a full quart. Measure the oil into the empty tank first, then add the fuel so the incoming gasoline stirs the mix.

Do I need to premix an oil-injected outboard?

Not in normal use. Mercury oil-injected motors, OMC VRO and VRO2 systems and Yamaha Precision Blend all meter oil into the fuel as the engine runs, and premixing on top of that doubles the oil going through the engine. The exceptions are break-in on a new or rebuilt powerhead, where many manuals call for premix in the first tanks on top of the injection, and a motor whose owner has disabled or removed the oil pump, which must then be premixed at 50:1 on every tank.

Can I use car 2-stroke oil in an outboard?

No. An outboard is water cooled and needs an oil certified to the NMMA TC-W3 standard, which covers marine lubrication, rust protection and how readily the oil blends with gasoline in a cold tank. The JASO FD oils sold for chainsaws and dirt bikes are formulated for much hotter air-cooled cylinders and are not certified for the marine requirements. Four-stroke car engine oil is never acceptable in either engine.

What ratio does a Yamaha 2-stroke outboard use?

Yamaha premix two-strokes are generally 50:1, and a number of the small portables carry a 100:1 decal, so read the cowl before you mix. Larger Yamaha two-strokes with Precision Blend oil injection are not premixed except during break-in. If the decal is gone, 50:1 with a TC-W3 oil is the safe default for a Yamaha of unknown spec.

What happens if I run an outboard too lean on oil?

The oil film on the cylinder wall thins, the piston skirt and rings pick up heat they cannot shed, and the motor scuffs or seizes. On a water-cooled powerhead it often shows up first as a loss of top-end power and a rough idle rather than an immediate failure. Too much oil is the far cheaper mistake: a smoky exhaust, a fouled plug and carbon on the piston crown, all of which clean up.

Outboard 2-Stroke Fuel Mix: Mercury, Yamaha, Evinrude

A two-stroke outboard is the one machine class where “just use 50:1” gets people into trouble, because the fleet on the water spans sixty years of different specifications. A 1996 Mercury 25 and a 1958 Johnson 18 are both two-stroke outboards, and their correct fuel mixes are more than twice apart. This guide covers what each brand asks for, how much oil that is in a real portable tank, and the oil-injection question that decides whether you should be premixing at all.

Start here: is your motor premix or oil injected?

Before any ratio matters, work out whether the motor meters its own oil. Three signs settle it in under a minute:

A separate oil tank on the motor or in the boat, usually translucent with a level line, means oil injection.
A second hose or a pump body between the fuel line and the powerhead on 1980s to early 2000s Johnson and Evinrude motors means VRO or VRO2.
A single fuel line and no oil reservoir anywhere means premix. Everything you burn has to be mixed in the tank.

If the motor injects its own oil, you fill the reservoir with TC-W3 and put straight gasoline in the tank. Premixing on top of a working injection system roughly doubles the oil the engine sees, which fouls plugs and coats the exhaust for no benefit. The detail on how these systems work is on the outboard mix page.

The common exception is break-in. On a new or rebuilt powerhead many manuals ask for a 50:1 premix in the first tank or two in addition to the injection, because the injection pump ramps oil with load and a fresh bore wants more than that during seating. Follow the manual’s break-in schedule rather than a forum’s.

The other exception is a motor whose owner has deleted the VRO. This is common on OMC motors from the era, because the pumps have a reputation for failing quietly and taking a powerhead with them. A deleted system means every tank is premixed at 50:1 from then on, no exceptions, and there is no warning light to remind you.

Ratios by brand and era

Brand	Era / model	Mix	Notes
Mercury / Mariner	Mid-1960s onward, premix	50:1	The long-standing Mercury spec for carbureted two-strokes
Mercury / Mariner	Pre-mid-1960s	25:1	Older powerheads, richer spec
Mercury	Oil injected, OptiMax, EFI DFI	Do not premix	TC-W3 in the reservoir; break-in per manual
Johnson / Evinrude (OMC)	1964 onward, premix	50:1	The year OMC moved the whole line to 50:1
Johnson / Evinrude (OMC)	Pre-1964	24:1	Often written 24:1 rather than 25:1; the oldest motors are richer still
Evinrude / Johnson	VRO, VRO2	Do not premix	Unless the system is disabled, then 50:1 always
Evinrude E-TEC	2004 onward	Do not premix	Direct injected, oil metered electronically
Yamaha	Premix two-strokes	50:1	A number of small portables carry a 100:1 decal, so read the cowl
Yamaha	Precision Blend oil injection	Do not premix	Except during break-in per the manual
Suzuki, Tohatsu, Nissan	Two-stroke premix	50:1	Oil injection on many mid and large models
Any brand	New or rebuilt powerhead	25:1 for break-in	Typically the first few hours, then back to spec

Two things about this table are worth saying plainly. First, the year matters more than the badge on anything older than about 1970, and a motor that has been repowered, re-cowled or sold on three times may not carry its own decal any more. Look the model number up by year before trusting a sticker. Second, where two sources disagree, the richer ratio is the safe side. Running 50:1 in a motor that wanted 25:1 is how powerheads die; running 25:1 in a motor that wanted 50:1 costs you a plug and some smoke.

How much oil that is in a real tank

Outboard tanks come in a small number of sizes, so the arithmetic is worth memorising for the two ratios you are likely to use.

Tank	50:1	25:1	100:1
3 US gal (11 L)	7.7 fl oz	15.4 fl oz	3.8 fl oz
6 US gal (23 L)	15.4 fl oz	30.7 fl oz	7.7 fl oz
12 US gal (45 L)	30.7 fl oz	61.4 fl oz	15.4 fl oz
24 US gal (91 L)	61.4 fl oz	122.9 fl oz	30.7 fl oz
25 L	500 ml	1000 ml	250 ml

A useful shortcut for the dock: a 32 oz quart of TC-W3 treats 12.5 gallons at 50:1, and a gallon jug treats 50 gallons. So a quart is almost exactly two fills of the standard 6 gallon portable tank, and if you are burning more than a quart a weekend you should be buying the jug.

For any tank size or ratio that is not in the table:

Fuel → oil Oil → fuel

Amount of fuel

Equipment (optional)

Mix ratio

Custom ratio (X : 1)

Add this much oil

12.8 oz

≈ 378.54 ml

Add 12.8 oz of 2-stroke oil to 5 gal of gas at a 50:1 ratio.

Marine oil is a different standard, not a different label

The oil in an outboard has to be certified to the NMMA TC-W3 standard. That is a licensed certification: the blender submits the oil, it is tested, and a passing oil carries a certificate number on the bottle. It covers the things a water-cooled marine engine actually faces, which are not the things an air-cooled chainsaw faces:

Lubrication at a much lower, thermostatically controlled cylinder temperature. An outboard powerhead is held cool by pumped water; an air-cooled cylinder is not.
Rust and corrosion protection, because the engine sits damp between uses and often for a whole season.
Miscibility, so the oil blends into cold gasoline in the tank rather than settling into a layer at the bottom.
Detergency at sustained mid-throttle, which is where an outboard spends most of its running life.

The JASO FD oils sold for saws and dirt bikes are built around the opposite problem, a very hot air-cooled cylinder, and they are not certified for the marine requirements. The full comparison is in TC-W3 vs JASO FD. TC-W3 oils are also ashless and usually dyed, which is how a marina picks the right jug off a crowded shelf at a glance.

The older TC-W and TC-WII marks are obsolete. If a bottle still advertises them, it is old stock, and there is no reason to run it when TC-W3 costs the same.

Ethanol is the outboard’s other fuel problem

Getting the ratio right does not help if the fuel underneath it has gone off, and a boat is the worst place to store gasoline. E10 pump fuel attracts water, and a vented marine tank in humid air gives it plenty. Once enough water is absorbed the fuel phase separates: an ethanol-and-water layer drops to the bottom of the tank, which is exactly where the pickup sits, and the gasoline left above it has lost both octane and most of the oil you mixed in, because the oil stays with the gasoline rather than the water layer.

That failure explains a lot of “it ran fine last season” calls. Practical defence:

Buy ethanol-free fuel where it is available, which in most coastal areas it is.
Mix what you will burn in a few weeks rather than filling a 24 gallon tank in April.
Use a fuel stabiliser rated for marine use in anything that will sit, and run the motor long enough to draw treated fuel through the carburettors.
Keep the tank full over the off-season to cut the air space that condenses water, and fit a water-separating filter between the tank and the motor.
If a tank has already separated, do not try to shake it back together. Drain it, dispose of the fuel properly, and start again.

Mixing the tank without making a mess

Premix is easier to get right in a portable tank than in a machine’s own fuel tank, because you can see what you are doing:

Work out the oil for the fuel you are actually adding, not the tank’s nominal size. A “6 gallon” portable holds a little over 6 when you fill it to the neck.
Pour the oil into the empty tank first, using a graduated bottle or cup rather than eyeballing a jug.
Add the gasoline. The incoming fuel does most of the stirring for you.
Cap it and rock the tank before the first start of the day.
Write the ratio and the date on a strip of tape on the tank. On a boat with two tanks and two motors this is the difference between a good weekend and a rebuild.

The kit that makes this repeatable, from graduated bottles to filter funnels, is covered in the measuring tools guide. The general procedure for any two-stroke is in how to mix 2-stroke fuel.

What a wrong mix looks like on the water

Smoke at idle, a fouled plug, carbon on the piston crown, a sooty prop hub. Too much oil, or an oil-injected motor that is also being premixed. Annoying, cheap, reversible.
Loss of top-end, a rough idle, a hot powerhead, eventual scuffing or seizure. Too little oil. On a water-cooled motor this creeps up rather than announcing itself, which is why people miss it until the compression test.
Runs, then dies after a few minutes, restarts, dies again. More often a fuel delivery or water problem than a ratio problem. Check the tank bottom for phase separation before you re-mix anything.
Fine on one tank, bad on the next. Almost always two tanks mixed differently, or one of them mixed by somebody else.

Quick reference

Premix motors from the mid-1960s onward: 50:1, which is 2.56 fl oz per gallon or 20 ml per litre.
Pre-1964 OMC (Johnson, Evinrude): 24:1. Pre-mid-1960s Mercury: 25:1.
New or rebuilt powerhead: 25:1 for break-in, then back to spec.
Oil injected, VRO, Precision Blend, E-TEC, OptiMax: do not premix except where the manual’s break-in schedule says to.
Oil: NMMA TC-W3, never a JASO air-cooled oil and never car engine oil.

For printable tables see the 50:1, 25:1 and 100:1 pages or the full mix charts. The outboard page covers premix versus oil injection and marine fuel handling in more depth, the 2-stroke mix chart has every ratio in one place, and the best 2-stroke oil guide ranks picks by use case including marine.