Two-Stroke Premix: Getting the Ratio Right on Older Outboards

There are a great many older two-stroke outboards still working hard in South Australia — Johnsons and Evinrudes, Mercurys and Mariners, Yamahas and Suzukis from the seventies through to the early two-thousands — and every one of them depends on the owner getting a simple sum right at the fuel bowser. Get the premix ratio wrong in one direction and the engine smokes and fouls its plugs. Get it wrong in the other direction and it seizes.

What does the ratio actually mean?

A two-stroke has no sump. The oil that lubricates the crankshaft, the bearings and the cylinder walls is carried in with the fuel, burns with it, and goes out the exhaust. The ratio is simply fuel to oil by volume: 50:1 means fifty parts petrol to one part oil, which works out at 20 millilitres of oil for every litre of fuel. Twenty-five litres of fuel takes half a litre of oil.

The common ratios you will see specified:

• 50:1 — 20 mL per litre. The standard for most outboards from the eighties onward, and the one most owners know.
• 100:1 — 10 mL per litre. Specified for some engines with modern oils, and what a lot of oil-injection systems deliver at cruise.
• 25:1 — 40 mL per litre. Older engines, and the running-in ratio the makers often specified for new or rebuilt powerheads, where doubling the oil for the first hours protects fresh bearings.
• 16:1 or 24:1 — the really old ones, from the days of thinner oils and looser tolerances. If your engine is from this era, the manual is the authority.

The ratio for your engine is not a matter of opinion and it is not a matter of what the last owner said. It is in the owner's manual, and we have collected the premix ratios for hundreds of outboards alongside the crankcase and gearcase capacities on our oil capacity charts.

Which oil

Outboard two-stroke oil is a different product from the two-stroke oil sold for chainsaws and mowers. The marine standard is TC-W3, formulated for engines that run cool and wet, at sustained load, with the exhaust dumped into the water. Air-cooled two-stroke oil is made for engines running much hotter and it does not protect an outboard properly. Look for TC-W3 on the bottle. The two-stroke oils in our Penrite range are labelled accordingly.

Too much oil, too little oil

Too little is the one that costs money. The crankshaft bearings and the piston skirts rely entirely on the oil that arrives with the fuel, and an engine run at 100:1 that was specified for 50:1 is running on half the lubrication it was designed for. It does not fail immediately; it runs a little hotter, the bearings pick up, the piston scuffs, and eventually it seizes, usually at full throttle a long way from the ramp.

Too much is the one that annoys you. Blue smoke, fouled spark plugs, carbon building up on the piston crown and in the exhaust ports, a rough idle and an engine that loads up on the troll. It is far less harmful than too little, which is why the old advice when in doubt is to err rich. But there is no need to be in doubt.

Mixing it properly

Oil into the tank first, then the fuel on top of it, so the fuel going in does the mixing. If the tank is already full, add the oil and rock the tank. A measuring jug or a marked squeeze bottle is worth having in the boat; guessing by eye is how engines end up on the wrong ratio. Mix only what you will use in the next few weeks. Premixed fuel goes stale exactly as petrol does, and the oil separates out over a long enough sit — our guide to stale fuel and why outboards will not start in spring covers what that does to a carburettor.

Separate fuel tanks and jerry cans, marked for premix, are in jerry cans and fuel tanks. Keeping a dedicated tank for the two-stroke stops the day someone fills it with straight petrol for the four-stroke.

Oil-injection engines

From the mid-eighties a lot of outboards moved to oil injection — Evinrude and Johnson VRO, Mercury AutoBlend, Yamaha Precision Blend and the rest — where a pump meters oil from a separate tank and the fuel goes in straight. Those systems vary the ratio with throttle, leaner at cruise and richer under load, and they work well when they are maintained. They do not work well when the pump diaphragm has aged or the oil line has cracked, and the failure mode is a seized engine with a full oil tank. A lot of owners of older injected engines have disconnected the system and gone back to premix at 50:1 for exactly that reason. It is a legitimate choice; just make it deliberately, and make sure the no-oil alarm is not the only thing standing between you and a rebuild.

If you own one of these older engines and want it looked over — the oil pump checked, the fuel system gone through, the plugs read — that is bread and butter at the Kadina workshop through outboard servicing and repairs. Ring 0412 532 984.