5 hp Mercury Outboard problem

[Sea Devil]

Got the dinghy out for the first time yesterday and stuck the Mercury 5HP on the back which has worked wonderfully for 5 years and it ran very fast - out of gear...

Cleaned the float chamber and adjusted the idle speed but as soon as I put it into gear the engine would die.... It will however run at very high revs if not in gear without load.... Would run astern more easily for a while then die. If I put it in gear and rev hard it would run at highish revs but die after a while...

The CD manual which is excellent value has been helpful but there is no 'fault finding' section in it... Any help - advice gratefully received
Michael

 

[cliff]

Slow running jet blocked?
Open the carb and give it a good soaking in carb cleaner.
--------------------

"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>

 

[steve_cronin]

I would need to inspect the engine to be more definite but since you have stated "... Any help - advice gratefully received" (although help offered on this basis usually attracts scorn and derision rather than polite disagreement, from the ever increasing numbers of arrogant- we all know who they are, self appointed experts on these forums) - I will offer you these thoughts to ponder on:-

If this particular carburettor has a "by-pass" method of providing an idle setting then maybe this bypass tube or drilling is blocked" This was quite common in OMC outboards of the 70s & 80s.

If there is a seperate slow running (& hence idling) jet tube, then after the length of service you state, this too could be blocked and the only way of clearing it easily is by using a single strand of wire extracted from a wire brush and gently clearing the hole with it.

The alternative area for consideration is the ignition system which, by the age of the motor must be a sealed capacitance discharge (or electronic) ignition mudule. These are energised still by the old tech method of a flywheel magneto, a magnet attached to the inside rim of the flywheel passing the core of a solenoid (electro-magnet). This can have become efficient so that sufficient energy is only produced at high revs. Corrosion is the main culprit in this resepect. Or the villain could be the CD unit itself - which would need replacing.

I will not make any rash statements like "Certain to be the Module" without seeing it but on this peaceful Sunday morning as I run through the results of yesterday's RNLI Flag Day, I offer these thoughts in the spirit in which they were requested.

Steve Cronin

 

[VicS]

From the symptoms you describe I would ceratinly be looking at the carb and the fuel system in general.
Although not ruling it out altogether I doubt if it is a problem with the ignition system. It would probably work or not work if it were the electronics. You have presumably cleaned and set the plugs assuming they are cleanable and settable types, inspected the HT leads and verified that you get a good fat spark at the plugs. The only way to check the electronic module would be by substitution if you suspect a problem there.

So then back to the fuel system. It is very likely to be the carb in need of a clean but before you wade into that check the fuel sustem in general. Filter & tank vent. If it has a remote tank the fuel pump and in general for leaks allowing air ingress, particularly at the fuel connector. If you do have a remote tank try pumping the priming bulb to see if that keeps it going.

Cliffs suggestion of soaking the carb in cleaner might do the trick if it is gummed up but most likely you will need to strip it down and clean though all the passages and jets and emulsion tube with a wire. I would not use a wire brush bristle, not a steel one any way, as that could damage brass jets etc. It may not be long enough for some parts any way. I would use a soft copper wire eg fuse wire or a strand taken from a piece of cable. Some people will say don't use wire at all.

If you do dismantle it make sure you work somewhere where you can find any little bits you drop and work carefully checking for small O rings and washers etc as you go. Lay the bits out in the order in which you remove them so that you know which order they go back together. All but the crudest of carbs will have some form of slow running "passage" it may or may not involve a separate jet and there may or may not be a separate adjustment for the slow running mixture.
Note carefully the setting of any adjustment screws before you start dismantling.

A diagram of the carb will be of immense help if only to ensure you have not lost any bits and to help put it back together properly.

Also flush out the fuel tank and if you have been using old fuel replace it with fresh.

I have this site noted as a source of downloadable Mercury manuals.

Finally I recommend the iBoats forums for help and advice on outboards. You'll find a forum dedicated to Mercury and mariner engines. If you go there be sure to quote the full model details (make, HP, year and model number)

PS are you sure the engine is not breathing its own exhaust fumes in some cases a failed seal at the bottom of the casing can cause that. That's unlikely in your case I guess, but check that the carb float is not punctured and that its associated needle valve is not dirty or sticking resulting in the carb fooding. Shutting off the fuel (or disconnecting a remote tank) once started will aid diagnosis of a problem there.

Sorry a rather long and rambling post but hopefully you will find something of help.

 

[Lakesailor]

All the above and has anyone mentioned an air leak (between carb and engine, thus increasing the air/fuel ratio). Racing without load and no power under load sounds very much like a very weak mixture.

 

[castaway]

My Johnson 7hp had a similar problem caused by the rather neat fuel pump which ran of crankcase pressure thro a tiny bleed hole from the c/case into a little diapragm pump.

On mine the diaphragm had developed a split..nice easy one that!

Regards Nick

 

[VicS]

[ QUOTE ]
an air leak between carb and engine

[/ QUOTE ] Another thing to check but it is not likely to have developed a leak during a winter layup. Anyway "racing" is not quite the same as "will run at high revs".

[ QUOTE ]
the rather neat fuel pump which ran of crankcase pressure

[/ QUOTE ] Thats how they normally work on small outboards. If it has an integral tank a fuel pump is unlikely but it is another possiblity with a remote tank. Pumping the bulb, or raising the tank above the engine should give some clues that something is wrong in that area if it has does have one. Also having filled the carb with the priming bulb it would run normally for a bit until the level fell.

I'd still go with a blocked slow running jet or similar.

 

[Sea Devil]

I use the internal tank 90% of the time - not been on a distant anchorage in ages!

Thank you all very much.. When I go down there on Tuesday I will strip the carb out and buy some carb cleaning fluid (if I can get my French around the concept) and find a really thin wire and get to work quietly and carefully on it..

It was very kind to give me the benefit of your knowledge..

thanks again

Michael

 

[steve_cronin]

Michael,

It COULD prove to be the slow running jet - which I suspect although the last small Mercury I had and my current mariner (same thing) don't have slow running jets but yours being slightly bigger might. In which case, my Dear Sir, when it is sorted out, you should remember that at the end of the season, following flushing the cooling system through with fresh water, it pays to "Stop it on Oil". That means run it on the very last dregs of petrol in the integral tank but just as it is emptying out, pour some neat two stroke oil into the tank and let it draw this downinto the carburettor. Increase the revs to med-fast before it stalls so that the momentum draws oil into every part of the engine giving it a thorough coating and ensuring that the carb. cannot suffer from corrosion since it is then swamped in oil. The motor will stop when it is saturated and then be ready for the winter of inactivity. It will be easy to flush out next season just by filling the tank with fresh fuel. You might have to clean the spark plug though but that is no chore compaired to what you are now experiencing!.

This used to be standard OMC training school practice.

Steve Cronin

 

[Sea Devil]

Steve hi,

Thank you for that... took the carb off again this afternoon and did the clean and wire bit up the jets,, and had a general cleaning.. looks good and is running good on the 'rail' - clamps on the stern. Will get it back on the dinghy on Friday I hope and make sure... Very grateful for the tips... must confess whilst living aboard I would of course never lay up as the motor was used every day... The recent change of circumstances has caught me out - I am almost surprised the thing is working at all... Your advice is gratefully received

Michael

 

[VicS]

[ QUOTE ]
is running good on the 'rail'

[/ QUOTE ] Do you mean you are running it out of the water. If so you will now need a new water pump impeller!