Why Your Outboard Won't Start in Spring: Stale Fuel, Ethanol and Stabilisers

Every spring the same phone call: it ran perfectly when it went into the shed, and now it will not start. Nothing has been touched. Nothing has changed. Except that five months have gone by, and five months is plenty of time for petrol to turn into something an engine does not want.

Why does an outboard refuse to start after winter?

Usually because of what happened to the fuel while nobody was looking. Modern petrol is a blend of light, volatile components that give you the easy cold start and heavier fractions that carry the energy. Leave it sitting in a vented tank and the light ends evaporate first. What is left starts slightly harder every week, and after a few months the lighter parts are gone and the heavier parts have begun oxidising into gum and varnish. The fuel still looks like fuel. It just does not behave like it any more.

In a carburettor engine, that gum settles exactly where you least want it: in the idle jet and the main jet, passages the width of a hair. A carburettor with a partly blocked idle circuit will fire on the choke and die the moment you take it off, which is the classic spring symptom.

Ethanol makes it worse

E10 is sold at plenty of servos, and it is legal and fine in an engine designed for it. The problem is what it does in storage. Ethanol is hygroscopic: it draws water out of the air. Over a humid coastal winter a partly filled tank breathes moisture every day, and the ethanol collects it until the fuel reaches saturation and the water drops out with the ethanol in it. That layer sits at the bottom of the tank, which is exactly where the fuel pickup is.

The engine then gets a slug of water and alcohol on the first start, and no amount of cranking helps. Ethanol also attacks older fuel hose, primer bulbs and some carburettor components, which is a separate problem with the same symptom.

For an outboard that sits between trips, plain 91 or 95 without ethanol is the safer buy. It is not about performance; it is about what the fuel does in the tank in the weeks it is not being used.

Stabiliser, and why it goes in before storage

Fuel stabiliser slows the oxidation and keeps the light fractions from leaving. It works, but only if it is in the fuel before the fuel starts to degrade — adding it to a tank that has already sat all winter does nothing for the varnish that has already formed. The routine is: stabiliser in the tank on the last trip of the season, run the engine long enough for treated fuel to reach the carburettor or injectors, then store. Penrite's Pro Series Petrol Fuel Boost does that job and also cleans as it goes, which suits the older engines that have built up a bit of deposit.

Diesel has its own version of the problem: not varnish but bug, a biological growth at the interface between fuel and the water that collects at the bottom of the tank. That is covered in our guide to diesel fuel filters and water separators.

Working through a no-start

If it is spring and the outboard will not go, work from the tank forward rather than pulling the plugs first.

• Fresh fuel. Drain what is in the tank, or at least in the portable tank, and put new fuel in. This alone fixes a large share of them.
• Primer bulb. Squeeze it. It should go firm in a few pumps. If it stays soft, either the bulb has perished, a hose has cracked, or the tank pickup is drawing air. A bulb that has gone hard and will not squeeze at all has usually had ethanol at it.
• Filter. Change the fuel filter. On an outboard that is often a small in-line element, and after a winter of stale fuel it will be holding whatever came out of the tank.
• Plugs. Now the plugs. Wet plugs mean fuel is arriving but not lighting: check spark. Dry plugs mean it is not arriving at all: back to the fuel side.
• Carburettor. If fuel is arriving and spark is present and it still will not run cleanly, the jets are blocked. Sometimes a carburettor cleaner through the intake will free an idle jet; often it is a strip and clean.

Hoses and bulbs, tank fittings and filters are in marine fuel hose and primer bulbs and the broader fuel system parts range. If the hose on your engine is more than a few years old and has seen ethanol, replacing it is cheap insurance against the next spring.

When to stop and ring

A carburettor strip is a fiddly job, and on a multi-carburettor outboard it is a long one. If fresh fuel, a new filter and a working primer bulb have not got it going, the sensible move is to hand it over before more cranking flattens the battery and floods the cylinders. We do this every week from September on at the Kadina workshop as part of outboard servicing and repairs. Ring 0412 532 984 and we will book it in.