Servicing Classic Outboards

I thought I would write a short(ish) guide to caring for simple outboard engines, based largely on what we do to service an engine before putting it on sale. It is really important to make sure your engine is in tip top condition, especially if you take your boat out to sea. We strongly recommend servicing your engine or engines at least once a year, it will ensure they are there when you need them and will give you peace of mind. We can always service engines for you we have all of the specialist tools needed to do the job, but as a guide, the following should give you a pretty good idea of what needs to be done. I would say at this stage that if you are not sure of what you are doing always seek professional advice. Outboards get hot and use petrol and electricity, which can be a very dangerous combination, always take appropriate safety measures and if in doubt, leave it alone and take it to somebody that has the right tools and expertise to do the job safely.

I usually start by having a really good look all over the engine, I particularly look for any signs of fuel spillage from split pipes etc and / or oil leaks around the lower unit, I pay particular attention to the condition of any fuel lines – a fire on a boat is not something you ideally want. If you have a remote tank check it all the way from tank to carburetor. In a remote tank set up you can get fairly high pressures the engine side of the primer bulb so make sure all connections are sound and that the piping is in perfect condition – any doubt replace it. Make sure that there are clips at every point where petrol pipes terminate, I like to use the spring type clips these days but small jubilee clips or even decent cable ties will suffice.

Before even starting to look at the mechanical or electrical bits and bobs the first job should to have a sniff of the fuel tank. You would be amazed at how quick modern unleaded stuff goes bad and it smells horrible when it goes off. If it is really smelly and not at all like fresh petrol, drain it off, dispose of it safely and put some nice new stuff in there. The other common fuel related problem with 2 strokes is where the 2 stroke oil settles out and gums up the carbs. On a lot of the smaller engines you have no choice other than to take the carb apart and clean it. On most larger engines the carb bowls will have drain screws in them. Undo the screw and use a suitable receptacle to catch any spilt oil or fuel. Let it run until it looks like decent 2 stroke mix then put the drain plug back in, repeat for each carb fitted.

While you are doing a general inspection check for any missing nuts or bolts, check all of them for tightness. Be aware that American engines use a lot of UNC and UNF thread forms whereas Japanese / Chinese engines use metric. A lot of the bolts may look similar but are not interchangeable. While we are at it with the fuel system, check any tanks for signs of distress or leakage, also check the fuel taps, which usually have a seal that is compressed with a gland nut – they are a favourite for coming loose and leaking or the packing gland gets worn out and causes a small leak. Check any filters, clean or replace depending on the type, again check for leaks.

Check the operation of the choke and throttle controls, make sure any interlocks are working correctly – many engines have a simple mechanical device to ensure that the throttle can't be fully opened if the engine is in neutral, some also have interlocks to prevent them going in to gear unless you are at idle. On no account be tempted to bypass any such devices fitted to your engine, they are there for an extremely good reason – mainly to stop you from starting the engine at full throttle while it is in gear. While that may earn you £250 on You've been framed such a thing could prove fatal, especially on a big and beefy engine!

OK, so next job is to whip out the spark plugs, they can tell you a lot about how your engine is running. There are many web sites that will show you what a plug should and should not look like but as a general guide they should be dry, lightish grey in colour and not show any signs of distress to the electrodes or the porcelain insulator – if in doubt change them out. If the plugs look like they have a whitish deposit your engine is running lean and the cause needs to be investigated before serious damage occurs. More often than not a lean condition is caused by an air leak on the intake side of the engine, it can also be down to ignition timing being too far advanced. If an engine is lean it will run far too hot and it's not unheard of for pistons to melt, which is often terminal. If the plugs are black and sooty then the engine is running rich, suspect either blocked air bleed jets or a choke problem.

Before you put the plugs back why not bung new ones in there? They only cost £3-4 each and it's no more effort to put new ones in than putting the old ones back. At least that way you have nice new seals and piece of mind. Make sure you replace like for like, a lot of the plugs originally fitted to the older engines are no longer available but there will be an equivalent. We use an awful lot of NGK BH6HS plugs these days, they seem to work well in a very wide selection of older outboards. Check on line to see what plug your engine used originally and only use a manufacturer's recommended replacement. A lot of people at this stage would advise checking the ignition timing but you can't typically do this on older engines as the timing changes as the throttle is opened and closed. What is important is to make sure that the carburetor opening and the ignition timing are synchronised so that for any given throttle angle the timing and fueling setting match each other. The points gap is also important, as is ensuring the points are nice and clean, always clean them before adjusting them. With a lot of the older engines you have to remove the flywheel to get at the coils and points, you must use the correct tool for doing this, do not use the sort of puller that grips the edge of the flywheel. Most OMC / Johnsons and Evinrudes need a three bolt puller that bolts to the flywheel, a large central bolt applies pressure to make the flywheel pop off. I made my tool myself, it is worth it's weight in gold as I can usually remove a flywheel in about 30 seconds in total safety.

While messing about with spark plugs it's a good time to check the compression. Only way to do this is with a proper tester, check all cylinders. Most manufacturers do not give an absolute value for what the compression should be but all cylinders should be within a few pounds of each other. Some engines run much higher compression ratios than others, for instance the old Albin four stoke 0-11 only runs about 40 or 50 psi, the late model high efficiency 2 strokes may run 110-120. Always check compression with the throttle wide open or you could get a false low reading. Low compression will result in an engine that is hard to start and lacking in power. If compression is down on one or more cylinders you are in for a big bill, if your engine is old a replacement may be in order. We try to hold a reasonable stock of good used power heads for just such an eventuality

Check the condition of all HT and LT leads, check the connections are all tight, give the condensors a quick visual check while you are at it. Finally use an HT tester to make sure the spark on all cylinders is totally healthy. Pay attention when doing this these beasts can give you a hell of a kick, especially the CDI ignition engines. I have seen a Chrysler CDI produce a spark of over 1 inch long – that would certainly sting a bit if you got in the way of it.
Next thing to check over is the cooling system. Most outboards are water cooled, typically they have an impeller type pump down in the gearbox which pumps water around the cylinder(s) before it is expelled through the tell tale or through the exhaust. If your engine is not pumping properly it. will overheat and let you down. Most common faults are either blocked inlets or worn out impellers. If you need to replace your impeller be aware that you also probably need to replace any gaskets that have had to be disturbed to get to the impeller. The worst thing you can do to an impeller is to run it out of water. I have seen them completely destroyed and unable to pump any water at all, it might just be me but Mariner and Yamaha ones seem to be the least tolerant to running out of water - don't do it on any engine, it's a bit of a pain to go changing them every 5 minutes/.
While we are looking at the gearbox we need to check out the seals. The typical box will have one or two seals on the propellor shaft, one on the drive shaft and one on the gear linkage. The only way to test these seals is with a hand pressure pump and gauge, the box needs to be pressurised up to about 10 PSI, it should hold that pressure for about 10 seconds without dropping much. If it won't hold any pressure or the pressure drops to zero within a few seconds, you have leaky seals. Pressurising the unit while it is underwater should lead you to the faulty seal but in all honesty it would be a false economy to not change them all while you are at it.

If you have got a leak you will most likely find that either the oil has gone milky or that it has leaked out, either situation requires immediate rectification. Next bit to look at is the starter system, this part is easy if you have a pull start, perhaps a little more involved if you have electric. If you have pull start, check that the recoil unit operates smoothly, if it does not then remove it and lubricate it before replacing it, adjust where required. Check the condition of the rope, if in doubt replace it.

If you have electric start you need to check your battery and charging system as well as the starter itself. If you have 12 Volt electrics your battery should be at about 12.8V at idle. The charging system should provide about 13 volts minimum at idle , rising to about 14.8 volts at full revs, this is just a guide line, it depends on whether you have an alternator or simple charging coils but it's about right as a guide. Check it with a good quality meter, check all terminals for tightness and check all wires for any sign of distress or rubbing. Make sure the connections to the solenoid are secure, also check any other terminals eg to the choke solenoid before giving your electrics a clean bill of health.

Next, check out all the grease nipples, look for hardened grease around anything that swivels, tilts or moves. Remove any hardened or grotty looking stuff with a good quality degreaser, dry it off and replace with nice new marine grade grease.

MOST IMPORTANTLY - check for the existence and correct operation of all the safety devices that the manufacturer fitted to your engine. Depending on when and where it was made it may have tilt locking device, start in gear protection, pull start lock out, safety covers or any number of other safety critical components, These have all been put there to protect the user and must always be in place and in good working order.
Final job is to give everything a thorough test, you will most likely need a test tank to do this on the smaller engines or flush muffs to do the larger engines. Never run a water cooled engine out of water, you will destroy the water pump impeller in seconds and undo all of the good work you have just done. With the engine being supplied with cooling water give it a test start and make sure it fires up correctly. If it is a 2 stroke and it wasn't correctly drained down after it's last use it may be reluctant to start, this is often due to the 2 stroke oil settling or gumming up the carb - you did follow the instructions at the top of this document - right?. If it's still reluctant to start a quick blast of easy start often helps matters here – another word of warning though. Two stroke engines need oil mixed with petrol, if you start it on easy start it will cause the engine to run dry and do a lot of damage to the bores and bearings. Only use it very sparingly, if it does not run properly investigate the cause, don't keep trying it on easy start you – have been warned!

Check the engine runs and picks up smoothly, check forward and reverse gears where fitted, check the engine rotates in it's transom bracket nice and smoothly, adjust the friction bearing if it is too tight or too loose. Check the engine lifts up ok, check it locks correctly when in reverse. Check there is a good flow of water out of the tell tale before giving the engine a really good clean and declaring it ready for a most enjoyable season. I like to turn off the fuel an run the engine until the carb is dry. This ensures that if I lay the engine down in the car I don't get a car that stinks of petrol, it also makes sure the fuel in the carb bowl does not gum up the jets and fuel control valve next time I come to use it.

I do hope this proves useful, do remember what I said at the beginning though, don't mess about with engines if you don't know what you are doing. There are experts like us in many places, particularly coastal towns. We are usually willing to help when asked and usually prefer that too having to put right bodged engines with broken bolts and the such like.

We have spent a lot of time and money acquiring the right tools to do all of the jobs required on your valuable engine, we also have the expertise to know how to service an engine correctly, if you don't fancy having a go yourself please call us on 01205 351698 to discuss your requirements.
We hope you enjoy your boating and look forward to welcoming both new and existing customers to our little business.
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