Stu Davies
“the engine whisperer”

Do you have a problem with your engine? Are you having issues with your fuel? Are your batteries coming to the end of their life? 

Read how I maintain my boat and the answers to common questions that are asked of me as the PBO  “engine whisperer”

Through hull valves

Made of a copper based material they used to last quite a long time. The issue with them is that some were and today are made of brass rather than the more expensive bronze. Brass is basically made of copper and zinc with trace metals added to make them more salt water resistant. The issue with them is when the zinc is eroded from the material and the copper emaining is then honeycombed and becomes weak and they fail. Nowadays manufacturers recommend changing every 5 years. My Beneteau was built in 1999, its thru hulls are mostly good, the valve material in those days was vastly superior imho. In the pic you can see one I have changed. No sign of dezincify-ing. It seized shut after 6 months on the hard in Portugal during Covid.

CO alarms are good for other things!

Because they pick  up battery gases as well

We got back on the boat. The CO (carbon monoxide) detector was pinging occasionally. However there was nothing on board being used that could produce CO.

I quickly checked the batteries in the detector at 1.49v and they were all the same, so I was puzzled as to the cause of the alarm going off

Then I noticed that the sticker that was on the bottom said replace the detector after 7 yrs and that it would start chirping at or around that date to warn to do so.

I assumed that was the cause of the noise so I took the batteries out and went to bed.

The next morning I did a recheck and confirmed the message to replace the detector and so removed it.

We went shopping in the morning and all seemed well although I did note that the BM1 voltmeter/ammeter was showing 13 amps charge which was unusual.

That afternoon I came down in to the saloon and smelt a familiar smell similar to 3 years ago when we had a battery charge runaway and we had changed the three service batteries. (We have 3 by 110amp batteries in parallel for the house bank)

It was “toasting” battery time! The BM1 was showing 14 plus volts and 13 amps charge! One of the batteries had failed and the chargers were thinking the battery was flat and so were wacking amps in to it.

I quickly switched off the mains charger and solar controller plus the battery switches, (they are in the port aft cabin, the last thing needed now was a stray spark near the batteries) and I carefully opened up the battery compartment in the stbd aft cabin. The end battery was hot, very hot and gassing. It was that gas (hydrogen and some hydrogen sulphide?) that was setting off the CO detector. It was interesting that the detector detected that gas even before we smelt it the next day.

I let it cool, then disconnected it and removed it. After cooling I put the voltmeter on it and it registered 6.9 volts.

A note of caution here, batteries explode if not handled carefully, if you are not confident about what you are doing, open all hatches and switch off the chargers and electric systems. Let everything cool down and get an expert in to deal with it.

The moral of the story? If your CO detector goes off and the internal batteries are ok, then a check of the main boat batteries might be in order! CO is not the only gas that can set them off!

Small outboards

Servicing a two stroke outboard.

We all have one, a small outboard to drive our dinghy.

Simple, cheap to run, seemingly indestructible. However their days seem to be numbered with legislation stopping their sale in the UK nowadays.

They unfortunately are “dirty” in terms of their exhaust, however, I would argue that in the grand scheme of things, their contribution of exhaust fumes to our world is not that great.

Keeping them running and looking after them is relatively simple

A little bit of history first though. The British Seagull was the fore runner

A simple robust and indestructible bit of kit, running on 16:1 2T mix (that was smoky!) Drop it in the sea and it would come up for more after a quick swill with fresh water.

No fancy magnet effect ignition on these! Just an old fashioned brass flywheel with magnets embedded in it and robust coils and a set of points to produce a spark. The carburettor was simple, easy to dismantle and clean.

However, it could be a cantankerous bit of kit with much winding of bits of rope and pulling to try and start it.

Much like the motorcycle industry it was overtaken by Japanese outboards and their derivatives. They were lighter and more reliable.

 Lighter, a big plus when it was being transferred from the mother boat to the dinghy.

They were bullet proof, advanced aluminium techniques and superior coils and magnets made them so. They originally had points but the superior coil technology used overcame some of the shortcomings of those.

Later they changed to magnet effect ignition systems that did away with points and their reliability and ease of servicing increased.

The carburettors were cleverly designed so that instead of having internal passageways to address low throttle settings, they used the throttle slide needle to control fuel flow at idle and so avoided the issues we have today with idle jets and their orifices becoming clogged with E alcohol related issues.

Yamaha, Tohatsu and Mercury became collaborators and rumour of one factory producing all of the small engines and badge engineering them spread!






I have used my Mercury 2.5 hp as the example for this article but the Tohatsu, Mariner and older Yamahas are near enough the same and others, the general principals apply.

Basically there is the power head with a spark plug, a flywheel with a powerful trigger magnet and coil which produces an impulse to fire the ignition coil. The carburettor is a simple device as described above.

The transmission is driven by a shaft from the engine power head to a right angle gearbox which drives the raw seawater pump and propeller. The pump is usually behind the propeller but on some of the bigger engines it is above the lower gearbox housing.

It is still driven by the shaft from the engine.

A service on these engines is basically a plug check, a transmission oil level check, a water pump impellor check and a carb check, along with a bit of lube on the bearing surfaces on the thrust side of the leg. The zinc anode on the leg also needs to visually checked. Usually held on by a 6mm bolt.

To check your spark plug, access it by dropping the inspection hatch as in the pic, and using a purpose made spark plug socket, they have a rubber insert to protect the ceramic insulator, undo the plug

Don’t forget, righty tighty, lefty loosy! The cylinder head is aluminium, the plug is steel, be careful tightening it up so as not to strip the threads.

The picture of my spark plug is a perfect example of a good plug, a clean burn and no oily deposits. I run my engine with a 50:1 mix, although at one time the manufacturers used to recommend a 100:1 oil mix. That recommendation gently disappeared, I suspect some customer problems helped with that! That plug is going back in for another season. If your plug is oily or carboned up, it can be cleaned with a brass wire brush. Some people say don’t wire brush them, I have done so for 60 plus years and have never had an issue after doing so.

Spark plug gaps and types varied slightly through the years and I have enclosed a screen shot of some of the recommendations. The plug I use at the moment is an NGK BR5HS which suits my set up perfectly.

Gapping the spark plug is part of the service and consists of placing a feeler gauge of the correct size, in my case 24 thousands of an inch, between the electrodes and gently tapping the bent electrode until it just grips the feeler gauge. For us older ones, a Woodbine ciggy packet was just the right thickness!!

The gearbox oil is a standard automotive hypoid gear oil and to check it the top screw in the pic is removed and it should be to that level. The bottom screw is to drain it and pump new oil in. It is the standard “remove two screws and pump it in till it overflows”

The water pump on my engine is behind a plate, behind the propeller. To access the impellor the prop is removed by pulling out the split pin and gently sliding it off the drive shaft. The drive is provided by a shear pin through the shaft which engages with a slot in the plastic prop. Be careful that it doesn’t slide out and drop in the oggin! Guess how I know about that? Remove that pin and then two 6mm bolts with 10mm spanner heads can be removed and the plate slid off to reveal the impellor. A thin nosed set of pliers can be used to gently tug the impellor out. There is also another pin through the drive shaft to drive the impellor. That is also a loose fit!

The water pump impellor can then be checked and as can be seen, mine, which has been in service for ten years or so, has distorted. This is caused by the impellor stopping with one of the vanes in the water transfer passage. This is going back in until the new one I ordered arrives. There are no cracks or sign of wear in it however.

People used to change their impellors every year in both the outboard and the main engine. I used to do that but after ending up with a pile of hardly used impellors in my “handy to keep bits” I decided to use the tried and tested method of check thoroughly and replace when necessary regime. It suits me. I check by robustly bending the vanes and inspecting the roots for cracks or wear, also the faces of the vanes where they rub the water pump faces. The metal bonding between the drive shaft and the rubber vanes is also carefully checked to make sure it hasn’t come adrift.

A top tip with water pumps is to clean the seal area at the back of the impellor, including where the seal spring is and fill it with silicon grease, this then helps to lube it on a dry start.

The impellor blades can go back in any which way, at the first turn of the engine they spring in to position.

Be careful when re assembling, the two bolts only need nipping up. They break easily!

The carb needs to be drained after use and if going to be stored for an extended time. The old way of turning the fuel tap off and running it dry is no longer valid in these days of alcohol fuel.

The float bowl must be emptied. The reason being is that ethyl alcohol is hygroscopic, it absorbs water, think whisky! If you run the carb dry it leaves a small amount of fuel in the float bowl. This then evaporates and leaves a drop of water in the bowl, this then reacts with the carb metals and causes jet blocking gunge.

The drain screw can be seen in one of my pix.

To finish off a light spray all over with WD40 containing lube oil or something similar and a wipe with an oily rag makes it look good and helps to protect the finish of the paint.

Winter storage:

If putting the engine away for the winter or long periods of time a simple regime to follow is as follows.

My engine is a direct drive engine ie no neutral gear. The bigger 3.5 engines have a gear shift and can be placed in neutral.

 So on my engine, when it is running, the prop spins.

I remove the prop as discussed previously and I get a bucket of fresh water in which I place the engine leg. The engine can then be started and run for a few minutes allowing fresh water to be circulated through out the engine cooling passages. A small amount of old fashioned glycol based anti-freeze can be put in the bucket, this contains anti corrosion products and will help to preserve the cooling system.

The fuel tap can then be switched off and the majority of fuel in the carb will burn off. THEN remove the drain screw in the carb float bowl to let it drain completely.

In the olden days, “fogging oils” were available to lube the engine for winter storage but my personal view is that todays hi tech two stroke oils are so good that this isn’t necessary. The engine stays lubed for prolonged periods of time.

Four stroke out boards:

They are now mandated because they allegedly produce less pollution than two strokes. They are basically the same as two strokes from the point of view of servicing plugs, the gear box transmission and the ignition system. However the carburettors are a different kettle of fish with internal pilot jet orifices and jets. These are so fine so as to deal with the tiny amounts of fuel consumed when ticking over that they soon become blocked if care is not taken with their fuel supply. I re commend that the more expensive fuels are used in these and that the fuel is kept scrupulously clean.

They are heavier than two strokes, a factor that needs to be taken in to account when older owners such as myself are transferring them from the mother boat to the dinghy. They have mechanical air intake and exhaust valves which have to be checked on a maintenance basis and of course they have an oil sump which has to be dealt with when transporting them on their side.

 Lay them on their wrong side at your peril, the sump soon lets oil trickle in to the cylinder. Basically the crank case breather lets the oil trickle in to the inlet manifold and then to the cylinder. Hydro locking then occurs and the spark plug has to be removed to expel the oil.

They are also vicious if the pull start isn’t pulled correctly, a nasty rap on the knuckles from the tee piece can easily occur!

Servicing and winterising is basically just the same as the two stroke except the valve clearances have to be checked now and again and the engine oil changed yearly.

My caveats for all of the above is that most small outboards are very similar and easy to service by the average skills reader. For any differences in a servicing regime, your engine handbook needs to be checked.

Small Outboard servicing