What fuel-to-oil mix ratio does my 2-stroke outboard need?

2-stroke fuel and oil mix ratio · Symptom: You need to premix fuel for a 2-stroke outboard and aren't sure how much oil to add, or you're seeing smoke, fouled plugs, or worry you mixed it wrong.

Quick answer: Use the ratio in your owner's manual. Most premix 2-stroke outboards run 50:1 (about 2.6 oz of oil per gallon of gas, or 100 mL per 5 L), while many current small Yamaha and Suzuki 2-strokes call for 100:1 (about 1.3 oz per gallon, or 50 mL per 5 L). New or rebuilt engines often need a richer break-in mix, commonly 25:1, for the first hours. Use only NMMA-certified TC-W3 outboard oil, never car engine oil, and shake the tank to mix it fully.

What goes wrong with the wrong mix (most to least common)

Too much oil: smoky exhaust, fouled spark plugs, and carbon buildup, especially when trolling at low RPM
Too little oil: poor lubrication that can score or seize a piston
Oil not mixed in: the engine gets uneven lubrication
Wrong oil type: car oil or mixing different oil brands (Tohatsu warns mixing brands can gel and block filter screens)
Premixing an oil-injected engine by mistake, or forgetting oil in a premix tank

Safe owner checks & fixes

Find the ratio in your owner's manual (or on the decal near the fuel connector or tank). Check whether your engine is premix or oil-injected. Oil-injected engines usually don't need oil in the gas, except during break-in if the manual says so.
Do the math: gallons x 128 / ratio = ounces of oil. At 50:1 that's about 2.6 oz per gallon (15.4 oz for 6 gallons). At 100:1 it's about 1.3 oz per gallon. In metric, liters x 1000 / ratio = mL of oil (20 mL per liter at 50:1).
Pour the oil into the tank first, then add the gas, cap it, and shake the tank to mix thoroughly.
Use only NMMA TC-W3 outboard oil. Direct-injection engines such as Tohatsu TLDI need a TC-W3 oil made for direct injection. Don't mix brands.
New or rebuilt engine: follow the manual's break-in procedure. Tohatsu, for example, uses 25:1 during break-in on carbureted 2-strokes, then back to 50:1. Expect extra smoke during break-in.
If you think you mixed it lean (not enough oil), don't run it. Drain the tank and remix. If you mixed it a little rich, it'll usually smoke and may foul plugs; remix the next tank correctly.

Parts / tools

Marked mixing bottle or measuring cup, TC-W3 outboard oil, clean fuel can, owner's manual.

Call a marine mechanic when

The engine ran on gas with little or no oil; it lost power, knocked, or seized; plugs keep fouling even with the right ratio; or an oil-injection warning or pump fault comes on.

Sources

Values not stated here are model-specific; check your owner's manual.