How To Start An Outboard Motor Engine

How To Start An Outboard Motor Engine

Small outboards mostly have a manual start which means you pull on a length of rope to turn the engine over. Fortunately the rope is on a recoil spring so it doesn’t have to manually re-wound onto a pulley on top of the engine after each pull.

To pull the rope, place one hand on the engine to give some resistance to pull against. Pull the ropes handle slowly to engage the pawl and then give it a swift steady pull.

When you do this the motor spins, the capacitor stores electrical energy generated by the flywheel and magnets and then releases that energy in a rush that leaps across the spark plug terminal to ‘earth’, producing a big fat spark as it completes the circuit.

It may take two pulls of the starter for the capacitor to gather enough energy to produce a spark so some engines will not start first pull.

When Is The Choke On Or Off?

It can be a bit tricky figuring out how to start an outboard motor engine.

What does ‘Choke On’ mean? How do you know when a choke is On or Off?

In the day of automatic chokes a manual choke can be confusing. Does the engine run with the choke on or off?

On the fuel side of things, if the carburettor is a diaphragm type, it will first need to be filled with petrol by pumping, with your finger, the clear flexible priming bulb until bubbles stop coming out the transparent fuel return tube. (Diaphragm carburettors pump fuel from the fuel tank into the carburettor. But they usually pump more fuel than is required so the surplus is diverted back into the fuel tank.)

Then put the choke on. “On” means the choke lever is pushed to one side so that the disc on the hidden end of it covers the air intake into the carburettor. If you are not sure which position this is, remove the air cleaner and look behind the filter.
How To Start An Outboard Motor Engine and how to work the choke

When the disc covers the aperture leading into the carburettor, the partial vacuum created by the piston as it moves down the cylinder, sucks more fuel than air into the combustion chamber at the ‘top’ of the cylinder. This is because the amount of air available is restricted by the disc. It’s likely that no more than two pulls of the starter rope is necessary to get sufficient fuel into the cylinder.

It is not actual petrol that is exploded by the spark but petrol vapour (gas) so if there is too much petrol in the cylinder head the liquid petrol will wet the spark plug and it will not explode. The engine is said to be flooded when this happens and the remedy is to wait until the surplus fuel evaporates.

The next move is to shift the choke lever to the ‘half choke’ position when the choke disc only half covers the air intake aperture. The engine will probably fire in this position on the first or second pull of the starter.

work the choke How To Start An Outboard Motor Engine

If the engine starts, let it run for a second or two at the most and then push the choke lever to the “Off” position so the disc is not covering the air intake into the carburettor at all. The engine will now be running normally although it may take moment or two to settle down and run steadily. Engines that have had an hour or two of running tend to run better than brand new engines.

This is due to the moving parts freeing up, the piston sealing better against the cylinder walls and diaphragm carburettors seem to work better after some use. I assume the diaphragm becomes a little more flexible after some use and sometimes a gummy residue is left inside the carburettor by the petrol used to test run new engines – this needs to dissolve away as it may block or partially block the fuel inlet jet or restrict the butterfly valve.

How To Start An Outboard Motor Engine using the choke

Most chokes work like this although there will be different designs and the lever that operates it may be different. The function will be the same.

If the carburettor has a ‘bowl’ under it, the chances are it is fed fuel by gravity so there will likely be an on/off tap on the fuel pipe or tube leading from underneath the fuel tank. Obviously you will need to turn the fuel on before the engine will ever start.

These carburettors do not need priming.  Turn the fuel off after use just in case the small valve under the carburettor, and concealed inside the fuel bowl, should fail to shut the fuel off and fuel will flow out into the engine once it stops running.

Most engines should start after about four pulls of the starter rope but many seem to have their own personality. For example, despite overhauls and new parts, my chainsaw takes six pulls before it fires just once on the seventh. Then I put it on half choke and it will start on ninth pull. Never fails – well, hardly ever.  Starting the chainsaw warms me up on cold mornings cutting firewood.

Tip: Let the engine run until it is warm before you start making it work hard at full speed.

Petrol: Use 91 octane petrol. Once the engine is run in you can often use 94 – check the handbook. I have no idea if it is safe to use 98 octane petrol in a small engine or not but it may make the engine run too hot or too fast as 98 octane petrol has significantly more ‘energy’ in it than 91.

This page  is unfinished – contributions, advice and corrections welcome.




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