Two-Stroke Premix Conversion: Living With an Engine That Had Its Oil Injection Removed

Find the model and serial number on the tag on the clamp bracket or transom bracket (on a PWC, check the hull and engine plates; see our engine serial number guide). Download or buy the owner's manual and, if you can, the service manual for that exact model. Look up three things: whether the engine was built with oil injection, the premix ratio for normal running, and the break-in ratio if the powerhead is new or rebuilt.

If you bought the boat used, don't trust a seller's "it's been converted to premix" without looking. With the engine off and cold and the cowling removed, check:

• The oil tank(s): removed, empty, disconnected, or still full? On many larger outboards there is also a remote tank in the boat.
• The oil pump area: is the pump still there, or is there a cover plate where it used to be?
• The oil lines: removed and the fittings properly capped, or just cut and left loose?
• The warning wiring: many injection systems have a low-oil or no-oil-flow sensor tied to the warning horn. Note whether its wiring was disconnected, taped or left hanging.
• Labels: any "premix" stickers on the cowl or tanks.

If you can't tell for sure, assume the engine needs premix and have a mechanic confirm how the conversion was done before relying on it. A half-finished job (open oil fittings, loose wiring, parts left in an odd state) can leak, suck air or confuse the warning system.

If the conversion was done cleanly and you're happy to mix every tank, you can keep running premix. If you'd rather have the factory system back, a dealer can tell you which parts are missing and what it takes to restore and bleed it. Never run the engine with the injection system half-connected while also using plain gas, and never assume a reconnected pump is working until a mechanic has checked it with the method in the service manual.

Use the two-stroke oil your manual calls for. For water-cooled outboards that is usually oil carrying the NMMA TC-W3 symbol; Yamaha and Mercury both point owners to TC-W3 for their two-stroke outboards. The NMMA certifies the additive package, registers oil brands every year and publishes the list of certified TC-W3 oils on its website, so you can check a store brand. Don't use automotive oil or two-stroke oil meant for air-cooled chainsaws or dirt bikes unless your manual specifically allows it. TC-W3 is a minimum, not the whole story: Mercury's oil guide, for example, sells a basic TC-W3 oil for low- and mid-horsepower two-strokes, a richer blend for high-horsepower engines, and a separate oil developed for its OptiMax and other direct-injection (DFI) engines. If your engine is a DFI model (OptiMax, E-TEC, HPDI and similar), use the oil its maker specifies; those engines shouldn't be converted to premix at all (see "Before you start"). Stick to one oil brand and type. Tohatsu, for example, advises against mixing different brands (or even different kinds from the same brand), because some combinations can gel and block fuel filter screens, starving the engine of oil.

Work out how much oil goes into the amount of gas you'll buy, using the ratio from your manual. Write the result on a card you keep with the gas can, for example: "X gallons of gas = Y ounces of oil." Use a graduated measuring container, not the "glugs" from a bottle. Too little oil risks scuffing; too much can foul spark plugs, build carbon and make the engine smoke and run poorly.

1. Set an approved gasoline can on the ground outdoors, never in a truck bed or on the boat deck (static can build up there).
2. Pour the measured oil into the can first, then add part of the gasoline.
3. Close the cap and shake or swirl to blend.
4. Add the rest of the gasoline and mix again.
5. Add marine fuel stabilizer at the rate on its bottle.

If you fill a built-in tank directly at a fuel dock, you'll have to add oil to the tank in proportion to the gas you pump. That's easy to get wrong; many premix owners switch to portable tanks for that reason. Whatever you do, mix only the amount of fuel you'll use within a few weeks, since gas starts to age quickly.

Mark the engine cowl, every portable tank, every gas can and the fuel fill (if you have a built-in tank) with a clear label such as "PREMIX ONLY: [ratio] TC-W3." Use a waterproof label or paint marker. This protects you on a tired day, and it protects anyone who borrows, services or buys the boat. Keep your premix cans separate from cans of plain gas for the lawn mower or car, and use a different color or a big label so they're never mixed up.

Find out from your manual or a mechanic what the warning horn on your engine still does after the conversion. On many engines the same horn also warns of overheating, so don't simply disconnect the horn. If the oil warning was disabled, you no longer have any alert for missing oil: your mixing habit is now the only protection. Ask a mechanic to make sure the overheat and other warnings still work (most sound briefly at key-on as a self-test; see our engine warning alarms guide).

For the first few outings after a conversion (or after buying a converted engine), keep an eye on how it runs:

• Smoke: a bit of light smoke at start-up is normal for many two-strokes on premix. Heavy blue smoke that never clears may mean too much oil.
• Spark plugs: oily, black or wet plugs, or frequent fouling during long idling or trolling, are common on premix because the engine gets the same oil-to-gas ratio at idle as at full throttle.
• Power and sound: any knocking, tightening up, sudden power loss or unusual metallic noise means stop the engine right away.

Run the engine only in the water, in a test tank, or (outboards) on external flush muffs with the water on before you start. Never run an outboard on its built-in flush port; that port is for flushing with the engine off unless your manual says otherwise.

A compression test after a season on premix gives you a baseline and catches trouble early. Our compression test guide explains how. Compare the cylinders to each other and to the spec in your service manual. Pull the plugs, too: their color tells you a lot about how the mixture is working.

Premix gasoline ages like any other gas. Add stabilizer when you mix, keep cans out of the sun, and use mixed fuel within the time your stabilizer and manual suggest. Before winter, follow your manual's storage steps for a two-stroke (usually fogging and stabilized fuel) and either run the carburetor dry or leave it full of treated fuel, whichever your manual says. Don't pour old premix into a four-stroke engine, a car or modern lawn equipment unless that equipment's manual allows it; take it to a household hazardous waste site instead.

Write the conversion details in your log: when it was done, by whom, what was removed, which ratio and which oil you use. If you sell the boat, give the buyer that information and the labels stay on. In BoatMend, note the conversion on the engine's record so your reminders and the AI Mechanic know it runs on premix.