Outboard Motor Won't Start? The Diagnostic Order That Works

Three Things an Engine Needs

Every petrol engine, from a 2.5 hp outboard on a dinghy to a 300 hp four-stroke on a centre console, needs the same three things to run: fuel mixed with air in roughly the right proportion, a spark at roughly the right moment, and enough compression to make the mixture burn hard when the spark arrives. Take any one away and the engine is a lump of aluminium.

The order follows from how often each fails and what it costs to check. Fuel fails most and is mostly free to find; spark is next and cheap to test with the right tool; compression fails least and needs a gauge. So: fuel, then spark, then compression. The boater who starts by pulling the flywheel off has usually skipped a closed tank vent.

The Checks That Cost Nothing

Do these in this order, out loud if it helps, before a single tool comes out of the bag. They account for a surprising share of dead outboards at the ramp and at the stern of the mother ship.

1. Kill cord in? The lanyard clip has to be on the switch, and the switch has to be in the run position. A surprising number of outboards are declared dead with the cord dangling from the owner's wrist.
2. Neutral? Most modern outboards have a neutral safety switch that stops them firing in gear. Push the lever firmly into the detent; half-way is not neutral.
3. Primer bulb firm? Squeeze it. It should go hard within five to ten squeezes and stay hard. How it behaves is your first fuel-system reading, covered below.
4. Vent open? Portable tanks have a vent screw on the cap. Closed, the tank goes to vacuum as fuel leaves and the engine starves after a minute or two of running.
5. Choke and fast idle? A cold engine wants the choke fully out and the throttle at the start mark. A hot engine wants neither. Getting this backwards is the commonest cause of a motor that started fine an hour ago.
6. How old is the fuel? Petrol with ethanol absorbs moisture from the air and can phase-separate, the water and ethanol dropping out as a layer at the bottom of the tank, within roughly 30 days in a humid, part-full tank. That layer is exactly where the pickup sits. Fuel that smells sour or looks hazy is finished.
7. Water in the fuel? If the boat has a fuel-water separator, look at the bowl. Water is heavier than petrol and sits as a clear layer with a visible line. Drain it. If there is no separator, a sample from the tank into a clear jar tells you the same thing in a minute.

Reading the Fuel Path

Fuel travels from the tank pickup, through the line and its quick connectors, through the primer bulb, through a filter, into the fuel pump on the engine, and finally into the carburettor bowl, where the float valve lets in exactly as much as the jets need. A blockage or air leak anywhere along that route shows up as a dry plug and a motor that cranks forever.

The primer bulb splits the route in half for you. Squeeze it and pay attention.

• Bulb stays soft after ten squeezes: the blockage or air leak is before the bulb. Suspect the tank pickup screen, a kinked or perished line, a quick connector not fully home, or a vent that is closed. Air bubbles in a clear filter confirm a leak.
• Bulb goes rock hard immediately: fuel cannot get past a point after the bulb. The carburettor float valve is stuck shut, the inlet is blocked, or an in-line filter is packed solid.
• Bulb goes firm normally, engine still dry: fuel is reaching the carburettor but not the cylinder. The jets are blocked. That is the gummed-carburettor case.

Which brings us to the whole story. A carburettor holds a thimbleful of fuel in its bowl. Leave the engine switched off with that fuel in it and the light fractions evaporate over weeks, leaving a brown varnish behind. The idle jet, a hole small enough that a strand of wire fills it, is the first to block. The result is a motor that pulls fine, has spark, has fuel at the bowl, and will not fire, or fires on choke and dies the moment the choke comes off. This is the classic failure on small two-strokes and four-strokes alike, and it is entirely a product of how the engine was stopped, not how it is being started. Cleaning it is a job of an hour with carburettor cleaner, compressed air and patience; the jets are brass, so no wire harder than the brass goes through them.

Spark: Pull the Plug First

A spark plug is a free diagnostic. Take it out and look at the tip.

• Wet with fuel: fuel is arriving. Either the engine is flooded, from too much choke or too many pulls, or the fuel is arriving and not being lit, which is a spark fault. Dry the plug, clip it into the tester, and crank.
• Bone dry: fuel is not arriving. Go back to the fuel path above. Testing spark on a dry plug wastes time.
• Black and oily: too rich, too much oil in a two-stroke mix, or a long spell of idling. Clean or replace, then start.

Test spark with an inline spark tester, a cheap tool that sits between lead and plug and shows the spark in a window. Do not hold the plug against the block with a screwdriver: the shock can damage the CDI, and a spark that jumps a millimetre to bare metal says nothing about whether it will jump a proper gap under compression. A good spark is a sharp blue snap. A weak orange flicker on all cylinders points to the stator or CDI; a missing spark on one cylinder of a multi points to that cylinder's coil or lead.

The kill switch and its lanyard

The kill switch works by earthing the ignition. If it is corroded, sticking, or the lanyard clip is worn so that it does not lift the contact fully, the engine has no spark and every other test comes back dead. Unclip and reclip the lanyard, spray the switch with contact cleaner, and if the motor fires with the switch bypassed at the connector, you have found it. Carry a spare clip: they are small, they fall overboard, and without one the engine will not run.

Electric start: clicks versus silence

On an electric-start outboard the sound tells you which half of the system has failed. A single click when you turn the key means the solenoid is trying and the battery or its connections cannot deliver the current: check for loose or green terminals first, then voltage. A resting battery below about 12.4 V is down; below 12 V it will not turn an outboard over. No sound at all means the circuit is open before the solenoid, and on most engines that is the neutral safety switch or the kill switch. Wiggle the gear lever. Our guide to marine battery life covers why a cranking battery left flat over winter rarely comes back.

Starts Then Dies

An engine that catches and then dies has just proved it has spark and compression. The fault is that the fuel supply cannot keep up once the bowl is used, or the circuit that feeds the engine at idle is blocked. Ask three questions. Did you push the choke in too soon? A cold engine needs it for ten to twenty seconds. Is the vent open? A sealed tank supports a minute of running and no more. Does it run on choke and die off it? Then the idle jet is varnished and the carburettor needs cleaning. If the primer bulb goes soft while the engine runs, the restriction is upstream: filter, pickup or a perished line sucking air.

Runs, But No Tell-Tale: Stop

Every outboard has a tell-tale, a small jet of water that should stream from the side of the leg or the cowling within seconds of starting. It is the only proof that the raw-water pump is turning and water is circulating. If the engine runs and the tell-tale is dry, switch off at once. First clear the obvious: a plastic bag or weed over the intake grille, or a wasp's nest of salt in the tell-tale outlet, which a length of wire clears. If the intake and the outlet are clear, the impeller has failed. Do not run it to see. The rubber impeller lives on water for lubrication and can be ruined by a minute of dry running, and an outboard overheats fast enough to warp a head before the alarm, if it has one, sounds. Impellers are a service item, typically every year or two, and the one on a tender that spends its life in sand and silt wears fastest.

Cold Start, Hot Start

Cold and hot are different procedures, and mixing them up is a reliable way to flood a healthy engine. Cold: vent open, bulb firm, neutral, kill cord on, choke fully out, throttle at the start or fast-idle mark. Hot: no choke, throttle a fraction open, pull. A hot engine choked is flooded before the first pull, and the cure for flooding is the opposite of everything you did to cause it: choke off, throttle wide open, and crank or pull to clear the excess. If the plug is wet, take it out, dry it, and give the cylinder ten seconds of cranking with the plug out before refitting.

The five-minute manual start, small outboard

For a pull-start outboard of 2 to 15 hp, the routine that works on nearly every make goes like this.

1. Leg down, intake in the water. Vent open, fuel tap on.
2. Squeeze the primer bulb until it is firm. On integral-tank motors with no bulb, skip this.
3. Neutral. Kill cord clipped on. Choke fully out. Throttle to the start mark.
4. Draw the cord slowly until you feel it come up against compression, then one firm pull through the full length. Do not snatch from slack; that is what breaks recoil springs.
5. One to three pulls should do it cold. If it fires, push the choke half in at once.
6. Choke fully in after ten to twenty seconds, throttle back to idle.
7. Look for the tell-tale. No stream, stop.
8. Six pulls with no result: stop pulling. Pull the plug, look at it, and go to the table above. More pulls only flood it.

Under five minutes, tell-tale included. Run it before the boat leaves the dock, as part of the pre-departure check, not once you are drifting towards the rocks.

Before You Pull the Cord

The pre-start checklist is short, and each item has a reason that is worth knowing, because the reason is what you remember on a rolling deck when the list is at home.

When It Is Not You

If the fuel is fresh and reaching the cylinder, the plug is new and the tester shows a strong spark, and it still will not fire, you have reached compression, and compression is the one thing on this list that a fresh tank and an afternoon will not fix.

A compression test is done with the engine warm if possible, all plugs out, throttle wide open, kill cord off, cranking for four or five revolutions per cylinder. The absolute figure varies enormously with the engine: many small two-strokes read in the region of 90 to 120 psi (6 to 8 bar), many four-strokes higher, and the manufacturer's manual gives the number for yours. What matters more than the absolute reading is the spread. On a multi-cylinder engine, cylinders within about 10 to 15 percent of each other are healthy; one well below the others points to rings, a scored bore or a head gasket on that cylinder. A single-cylinder reading far below the manual's figure is the same news. Either way the powerhead is coming apart, and unless you enjoy that, it is a shop job.

The other shop job is ignition beyond the plugs and kill circuit: the stator under the flywheel and the CDI that fires it. Diagnosing which has failed needs a peak-voltage adaptor and the manufacturer's tables. No spark on all cylinders, with plugs, leads and kill switch eliminated, is the point to hand it over.

Prevention: Stop It Right

Back to the thesis. Almost every gummed carburettor was made by switching the engine off with fuel in the bowl and walking away. The fix is a habit, not a part.

• Run the carburettor dry any time the motor will sit for more than a couple of weeks. Disconnect the fuel line or close the tap, and let the engine idle until it stops. Small motors with a drain screw on the bowl: open it and drain the last of it.
• Or stabilise the fuel and run the engine long enough to get treated fuel through the carburettor. Stabiliser does nothing sitting in the tank; it has to be in the bowl.
• Buy fresh fuel in small quantities. A 25 litre (6.6 US gal) tank filled in May and still half full in August is a carburettor cleaning waiting to happen. Ethanol-free fuel, where you can get it, stores far longer.
• Spare plugs and a spare kill cord aboard, plus a plug spanner, a small inline spark tester and a length of wire for the tell-tale. Together they fit in a sandwich bag and fix most of the table above.
• Store it upright, or on the side the manual says, so cooling water drains out of the leg and does not run into the powerhead.

The full end-of-season version, including fogging the cylinders and the impeller change, is in our guide to winterising an outboard. Do that in autumn and the spring start is one pull.

The Log That Tells You Which Row You Are In

Half of the table above turns on the past. When was the fuel bought? When were the plugs changed? Has the carburettor ever been cleaned? How many hours on the impeller? An owner who can answer in ten seconds skips whole rows; one who cannot pulls a good carburettor apart because it seemed likely. A maintenance log with a line per job — date, hours, part, what you found — is the cheapest diagnostic tool on the boat. The Marine Logbook app keeps the outboard's service history, fuel purchases and running hours on your phone, so the day it will not start you know whether the fuel is three weeks old or three months, and whether the plugs went in last spring or the one before. What you should be writing down, and how little of it there needs to be, is in what to record in a ship's log.

The Bottom Line

Fuel, spark, compression, in that order, and the free checks before any of them. Kill cord, neutral, vent, bulb, fuel age. Pull a plug and let wet or dry choose your branch. Test spark with a tester. Stop the instant the tell-tale is dry. And when the day is over, remember that nine outboards in ten that will not start were stopped wrong, not started wrong: run the carburettor dry, or stabilise the fuel, and the next start is one pull instead of an afternoon.