Servicing four stroke outboards
– How Douglas Hopwood does it

This article arises from an exchange on the forum following a request from a member about servicing his 5hp Mariner four stroke outboard. It relates specifically to the 5HP and 6HP four stroke outboards branded in the UK as Mercury, Mariner or Tohatsu (may also be marketed in the USA as Nissan). However, users of other motors of similar size may also find this useful although the list of items to be dealt with will differ as will the oil capacities and plug gap. It is less applicable to two strokes and is probably least applicable to older two strokes with ignition contact breaker points to consider.

On Douglas’s web site at www.hopwood.uk.net  under Drascombe Projects you will also find details (Previously published in DAN) of how to make an engine stand that may be useful to store your motor in the winter.

First post

I own a Mariner 5HP 4 stroke outboard engine. Up until now I have had the motor serviced at a dealer. Does anybody have any experience of servicing these motors and is it that difficult?

Reply

The most difficult bit of servicing these engines is finding out what comprises a service.

I have a 6HP Mercury (same engine - different max revs and different colour). The handbook is common to other engines and suggests silly things like replacing the fuel pump diaphragm and the water pump impellor every year!

If you have done a little car maintenance, then all the regular preventative maintenance items are easy but the pump impellor can be tricky to get at. However, if your engine is not running right them you have tuning issues and so you will need either a workshop manual or a (good) dealer for this service as there is no information about carburetter jet settings or ignition timing in the hand book.

I set out below the service schedule that I work to. Firstly though, you have to estimate your motor hours (I keep a log). In a car the schedule is usually every 10,000 or 12,000 miles or every year which ever comes around first. Cars have oil filters, our outboards don't. In the days before cars had oil filters the service interval was 1,000 or 3,000 miles. The outboard service interval is based on 100 hours which with the constant speed running of an outboard and modern lubricants I reckon is equivalent to about 3,000 car miles.

I do between 25 and 55 hours per year - virtually never flat out. So far I have clocked up about 160 hours in total. If you keep your boat in the water you may do more hours and need to consider varying the schedule.

Given the above, my annual procedure is as follows:

• Run motor (big plastic bin on bricks under motor and jockey wheel on trailer wound up to max).
• Check for strong flow of water from cooling water outlet.
• Allow time for thorough fresh water flush.
• Disconnect fuel and run motor dry. This is controversial since the lean mix just beore the motor stops is not beneficial but a carb full of last years petrol could cause poor starting next year. I compromise by running it dry at tickover speed.

Change:

• engine oil whilst engine still warm. (to get the max old oil out). A cheap measuring jug and a funnel kept for the purpose are useful for this job. 10W 30, 500ml
• I usually transfer the engine to the bench at this point where the rest of the procedure is carried out in the warm and at leisure (With the motor on a stout transom thickness bit of pine clamped firm in the vice)
• gear oil. Remember that this is done by squeezing the tube into the bottom hole until it comes out of the top one, then put the top plug back first. EP 90 195 ml
• - petrol. Nowadays I empty the tank into the car (not easy with the restrictor in the car's tank orifice for petrol station unleaded pumps) then top the car up with fresh petrol. The boat tank gets fresh petrol in the spring.

Oil:

• Tilt pivot bolt
• Tilt catch pivots
• Tilt latch pivots
• Gear shift handle detent DO NOT GREASE. If lubricated too well the motor will vibrate itself into gear from neutral! Just apply a very little oil so it will operate smoothly.

Grease:

• Co-pilot wing nut threads (This oddly named item is the wing nut that controls steering friction)
• Engine swivel grease nipples (2)
• Transom Clamp screw threads (2)
• Prop splines (take the prop off to do this)
• Wipe with grease any parts of the tilt mechanism that are vulnerable to corrosion where the paint has worn off.

Check:

• All electrical connections.
• Remove and check the spark plug for a good colour and gap. 35 thou or 0.9mm 20 lbs ft or 27 Nm if you have a torque wrench - otherwise think about going into aluminium and don't over do it. You might have to take it out again at sea! My plug is in good shape after 4 and a bit years and 160 hours. I might give it a new one as a treat next year. While the plug is out I squirt a little engine oil into the cylinder and turn the engine over on the pull cord.
• Check the pull cord for fraying.
• Feel for slipping caused by missing plastic teeth on the flywheel which the pull cord engages.
• Check the condition of the anti-corrosion anode.

Clean the engine:

• Wipe down all over
• Use a rag to dry any moisture on the power head
• Spray the power head with WD40
• Put the lid back on and wax it with car wax

Service the propellor:

• Check for major blade distortion, damage or cracks (Any serious damage will call for replacement)
• File smooth any small nicks
• Respray the prop if necessary

Occasional:

• Grease inner surface of tiller handle rubber bush every 4 years or so.
• Petrol pump diaphragm. The distributor says every 100 hours. The dealer says only if you have problems. Perhaps I will replace it at 6 years (250 hours for me). It looks fine at present.
• Water pump impellor. If it wasn't such a pain to get to I'd inspect it annually. (I would not trust a dealer to do it in a regular service unless the tell tale flow is low). The distributor says 100 hours. The dealer says 2 years. Again I plan to replace it at 6 years (250 hours for me). I have a strong cooling water flow at present. If you do high hours in a silty water environment you may want at least to inspect this every year.
• There is no mention of the valve clearances in the manual. With the hard valve seats used these days they shouldn't need attention until the equivalent of 50,000 car miles. As by my reckoning I have only done about 4,500 miles so far I shan't worry about the valve lash for a long time yet.

Hope this helps.

Second Post

Thanks for such a comprehensive reply. I do on average 12 hours a year and have had it usually serviced every year. I missed last year and in 2005 it cost me £175.00. It is an essential saftey device and as yest I have not made up my mind what I am going to do.

Follow up reply

If I was only doing 12 hours per year and was doing to do the work myself, I would still give the motor the annual service as below. It doesn't take long, uses very little oil and grease and it is a chance to check the motor over.

If I had to pay the £75 they charge locally I would not lose any sleep about skipping a year although I would still flush it, look it over and check the cooling water tell tale (which you should do every trip anyway).

If it lives on a mooring all summer then I would also do the little oil and grease chores on the pivots etc. grease the prop splines, wipe the leg down and WD 40 the power head. In fact I would do these mid season as well if the opportunity arose.

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