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HSH Yacht Articles

Outboard engines


Jeremy R. Hood

At times I was envious of the other cruising sailors with their smart new Avon sports boats and their large outboards enabling them to plane around the anchorage at high speed. In an instant almost they were in their dinghy and away, racing towards the dinghy dock or out to some reef to fish or dive. And there was I with my 2 horsepower Seagull outboard still on the stern of Melos as I rowed silently between the anchored yachts. I enjoyed the rowing and I certainly met more people as I passed slowly by their boats, yet sometimes the Seagull was pressed in to use and then, when I needed it, all too often it was not powerful enough to take me far and so eventually, and somewhat reluctantly, I got a larger one.

Outboard engines are strictly utilitarian beasts, rarely cared for by sailors yet expected to perform perfectly from one season to the next. Maybe yours is the main means of propulsion aboard your smaller sailboat or perhaps it is your dinghy motor. Whichever use you put it to, like me, you probably pay little attention to it until you have problems and then after a few choice words you rather belatedly find yourself having to spend more time fixing it then you ever intended. And so, after writing this article I'm going right down to service mine which hasn't been run for the last year or so!

How they work

Unlike inboard diesel or gasoline engines, small outboard engines are relatively simple and, apart from the means of achieving a spark, have changed little from models produced 30 or 40 years ago. They are all two stroke engines, similar to many smaller motorcycle engines in this respect, cooled by the water in which they run and started manually. Fig 1 shows the parts common to most small outboards.

Two stroke engines differ from the 4-stroke gasoline engine in your car or the diesel inboard on your boat because they have no valves to open and close in the cylinder head. Instead of valves they have openings in the cylinder wall (called ports) through which the fuel mixture enters and the exhaust gases escape. As the piston descends in the cylinder, a gas air mixture is forced in from the intake port and then, as the piston rises, the mixture is compressed and the fuel then ignited by a spark from the spark plug. The piston descends and as it does so it uncovers the exhaust port and the exhaust gases are pushed out. Whereas on a 4-stroke engine there are four separate "strokes", Induction, compression, power and exhaust, on a 2-stroke engine these are combined into induction/compression and power/exhaust. Hence the 2 "strokes". Understanding this difference will help in realizing why some of the problems occur with an outboard engine.

In order for the gasoline/air mixture to be forced into the cylinder, pressure is required. As the piston in an outboard engine travels downwards it creates pressure in the crankcase below and it is this pressure which is used. The gas air mixture is drawn into the crankcase and then with the build up of pressure as the piston descends it is forced through the intake port and into the combustion chamber. This system works fine but there is one problem. If gasoline is circulating around the crankcase what happens to the lubricating oil that you would expect to find there. Well, there isn't any! If there were it too would find its way into the combustion chamber and be burned causing no doubt clouds of black smoke. In a two stroke engine there is no way that oil can be left to circulate as it does in a 4 stroke engine and so the lubricating oil has to be added to the fuel. As the fuel passes through the engine, the oil lubricates the bearings before it enters the combustion chamber where it is burned along with the gasoline resulting in the characteristic smell of an outboards exhaust and the faint blue smoke so often seen.

Apart from the differences mentioned above, the other parts of an outboard engine are relatively familiar. If the outboard engine is small and has an integral tank then the fuel is fed to the engine by gravity. If an external tank is used then a small fuel pump is used for this. The outboards carburetor is extremely basic and similar to the ones used on automobiles of yesteryear. In the carburetor the gasoline and oil mixture are combined with air to form a gaseous vapor which then enters the crankcase as described above. The exhaust gases which are expelled from the cylinder are then mixed with the cooling water in a similar way to that used for the exhausts of marine inboard engines and the water/exhaust mixture is pumped out near to or below the waterline. The spark to ignite the gasoline is created in a way similar to that on a car though because an outboard usually is not connected to a battery, a coil or magneto generates the necessary electricity in a way similar to but more primitive than the alternator on your car.

Outboard engines run on their side. That is, rather than having a piston travel up and down it travels in and out allowing the shaft that it turns to be vertical. This shaft then extends down to the lower unit where the gearbox is situated. Here the vertical rotation is changed to a horizontal rotation to drive the propeller.

Servicing your outboard

Not knowing too much about outboard engines and certainly not having serviced my own as I should perhaps have done, I went to see Sonny and Dan Biggs of Biggs Brothers Marine who have been working on outboard engines for a combined total of 35 years. Sonny went through with me the main user serviceable items on an out board engine:

  1. Changing the oil in a lower unit.

    Because the lower unit of an outboard is small and can contain only a little oil it is necessary to change it regularly. Photo 1 shows a typical lower unit with Sonny's finger pointing at the lower unit oil drain screw. Remove this and check the oil to make sure that no water has got in (Photo 2 shows another outboard where water has entered the oil chamber as a result of a failed seal. The result is a creamy colored oil which has to be changed!). Photo 3 shows the upper oil level screw. Sonny recommends that oil should be filled from the bottom upwards (Photo 4) to eliminate the possibility of air pockets. Once oil emerges from the top screw it can be replaced along with the bottom screw and the job is done.

  2. Checking the prop

    After changing the oil in the lower unit it's a good idea to check the prop. Firstly remove the cotter pin and the retaining nut and then the prop itself. Now is the time to check for fishing line caught around the shaft which will damage the shaft seals extremely quickly. Remove any line and clean and grease the splined shaft (Photo 5). On older outboard engines the shaft was not splined but a shear pin was used to connect the prop to the shaft (Fig 2) and if yours has such a pin check its condition and make sure you have a spare pin aboard.

  3. Checking the water pump impeller

    If you have problems with the cooling water or have run the engine without water for more than a couple of seconds then it is likely the water pump impeller will have distorted or failed entirely. To get to this you need to remove the lower unit from the engine and inspect the impeller as you would on an inboard engine. Photo 6 shows Sonny pointing to the water pump.

  4. Servicing the upper unit

    Apart from undertaking major repairs there is really very little to do to the upper unit (Photo 7). However one important job is to replace the spark plug(s) at least annually. As Dan Biggs pointed out the engine is breathing moist and possibly salty air all the time and rust is extremely likely. They recommend new plugs every 100 hours or annually whichever occurs first. Another service item is the fuel filter seen clearly as the white unit in the foreground of photo 7. This should be taken apart and the internal filter checked and cleaned. And finally Sonny and Dan recommend coating the whole upper unit with a corrosion inhibitor such as Corrosion Guard, LPS 3, or Corrosion Block. This should be sprayed liberally all over the upper unit coating everything well paying particular attention to all electrical connections and fittings. Doing this alone could save you much time and money later.


If treated right, run correctly and serviced regularly your outboard should be as reliable as... Forget it. Who does all this anyway. So what do you do if problems occur. Firstly make sure the engine is in neutral and that the safety lanyard (if fitted) is attached. Without these two being done the engine is designed not to run! And then if it fails to start check for a spark. Remove the plug, connect up the lead and hold it against the engine casing as you pull the starter cord. Check first to see if you have a spark at all and if you do see what color it is. A yellow spark indicates poor current while a blue spark is ideal. If no spark occurs try a new plug before consulting a mechanic. Most modern outboards have electronic ignition circuits and without the correct test equipment or the ready availability of spares it is not easy to be sure which of the main components has failed.

If you have a spark but your engine still fails to run the problem is most likely in the fuel system. See if you have fuel to the carburetor first then if this is OK check to see if the spark plug is wet indicating fuel is reaching the combustion area. Sonny and Dan told me that most fuel problems originate with old fuel where the more volatile constituents have evaporated leaving a thicker varnishy fuel. Try new fuel and try cleaning the carburetor. But don't take off the carb and soak it in carburetor fluid as it is not designed for this and important seals may be destroyed. Instead use a spray from the outside. If you need to remove any of the jets then first screw them fully closed counting the number of turns required before removing them. This way you will be able to set them back exactly as they were.

Running your outboard engine

Just pull the starter cord and it will run! Well it will and it won't. Some outboards seem so docile and compliant that they run at the first yank on the cord while others have to coaxed and cajoled into action by a series of procedures akin to a ritual dance. I was helping the owner of a trimaran take his boat down to Bolivar recently where it was to be hauled out and the first part of the task was to get the outboard running. Without even trying to start it the owner removed the cover (an ominous sign) and tugged at a few moveable parts. Then he adjusted the choke lever extremely precisely to what seemed to me to be a precision of around 1/16". Next it was a squirt of starting fluid in the carburetor before he pulled the starting line. And believe it or not the engine did start! But after a few seconds it died and the whole procedure had to be repeated. Starting this wilful machine was not the only sickness it had but when it was in gear and running it seemed to do just fine... until one attempted to touch it!

Assuming that you have serviced you outboard recently, given it new fuel and a new spark plug then it should run just fine. And to keep it running this way there are a couple of things you should do before the end of each use. Firstly, run the engine at idle for a couple of minutes. The richer fuel mixture in this situation will contain more oil and your engine will receive a liberal amount of lubrication to coat all surfaces before you stop it running. And secondly, disconnect the fuel line or turn off the fuel supply and let the engine run until it dies. Fuel left in the carburetor or engine will evaporate leaving behind a varnish that can gum your engine up and block the narrow jets in the carburetor. Sonny and Dan Biggs told me that this is the single biggest cause of outboard problems accounting for some 90% of repairs!

When your outboard has been for a swim! Don't think it won't, because they all do sooner or later! So bearing this in mind your first step is to make sure that a safety rope is attached so that you can easily recover it when its underwater. Working for a charter company in the Caribbean a few years ago this was a regular occurrence and we soon became expert at resuscitating drowned engines. The first thing to do is to treat it immediately. Better leave it underwater than take it out then do nothing as rust will inevitably destroy the engine this way. Most times you will be able to get the engine running again but occasionally serious damage may have resulted. If water was sucked into the cylinder and the engine tried to compress the water (impossible) then damage to the piston or other components could have occurred. Best keep your fingers crossed and try to get it running. If you can't get the engine to a mechanic or you choose to do the work yourself here's a procedure to follow:

  1. Thoroughly clean the engine with clean fresh water

  2. Remove the spark plugs and turn the engine by hand to eject all water that will inevitably have been sucked into the engine. If the dunking occurred in salt water then use fresh water to flush out any remaining salt. When finally attempting to eject all water, manipulate the engine so that water caught inside may fall out!

  3. Use an air compressor if possible to dry the engine

  4. Remove the flywheel and clean and dry all electrical parts. Spray with Corrosion Block or similar.

  5. Squirt outboard oil liberally into the spark plug hole and rotate the engine to coat all surfaces.

  6. Dry the spark plugs thoroughly (in an oven if possible)

  7. Drain all fuel lines, filters and the carburetor. Flush with new gasoline. If the tank was immersed then do the same fore this.

  8. Prime the engine and get it running then leave it running for as long as possible. If it starts and runs then all should be well!

And now I suppose its time to follow my own advice and try and get me old Johnson 4 running again. Its been quite a while so I'm not sure what I will find. Perhaps if you see me around you should ask me about it and I may feel guilty enough to service it regularly in the future! But one last piece of advice from Sonny and Dan. If you do your own work on your outboard get hold of the factory service manual and read it!

Thanks to Sonny and Dan Biggs from Biggs Brothers Marine for their time, help and advice in preparing this article. Sonny and Dan may be reached at Bancers Marine 326 2287 or they may be paged 616 3060.

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