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  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?


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:





Clean the engine:

Service the propellor:


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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