Non start outboard

[jacaldo]

Hi, Don't know if anyone can help, I recently acquired a 2nd hand mercury 4hp 4 stroke outboard as a non runner. I have tried and checked the following: Good plug, compression good, lift pump good, removed stripped and cleaned carb, check for a spark which is weak but there, I don't expect a big spark off a magneto pulling it by hand, the plug gets wet and even if I spray brake cleaner direct into the carb it makes no difference. I have checked the key on the flywheel thinking it may have sheared and thrown the timing out but that is ok, I have tried the fuel from a remote tank and the engine tank. I do not have a spare coil to try and do not know if it has something like a cdi unit as part of the ignition system. Occasionally I can fire it up, it runs for a few seconds but will not rev up and it sounds very flat, also sometimes when trying to start it, it will backfire through the carb, which made me think of a timing issue, forgot to mention if I heat the plug up this may help it to start. The problem I have is with no spare parts I will have to trial and error repair, a rather expensive way of fixing things??? Any help or advise would be much appreciated, thanks in advance.

 

[VicS]

check for a spark which is weak but there,

A spark from a CDI should be capable of jumping about 7/16".

Brake cleaner ... what's in that ?

it runs for a few seconds but will not rev up and it sounds very flat, also sometimes when trying to start it, it will backfire through the carb,

sounds like carb not properly cleaned but always fit a new plug when having troubles as so often that cures matters even when symptoms suggests carb problems.



Always quote full model details when asking for advice on outboards

 

[Rigger Mortice]

Even on a recoil starter you should get a decent spark. On a quiet day you should be able to hear it "crack".

 

[30boat]

One carachteristic of old style magnetos is that the spark is at it's strongest at low revs.Don't know if it still applies to magnetos plus CDIs.

 

[DownWest]

First change the plug (as Vic says) It must be CDI, so expect a healthy spark. A feeble one might be the plug (and fails under compression) or the ignition coil.

 

[jacaldo]

A spark from a CDI should be capable of jumping about 7/16".

Spark is very weak, if it was on a car or motorbike I'd be very concerned

Brake cleaner ... what's in that ?

It contains a fast evaporating solvent that will run a car


sounds like carb not properly cleaned but always fit a new plug when having troubles as so often that cures matters even when symptoms suggests carb problems.

New plug fitted and the carb has been stripped and cleaned properly


Always quote full model details when asking for advice on outboards

I'm at home now so unable to get the model details, I thought this post for help could apply to most 4 stroke small outboards.

 

[CAPTAIN FANTASTIC]

The spark has to be able to jump across at least 5 mm; it sounds like issues with the cdi set; they are expensive to replace, however it is worth it.

 

[Lakesailor]

I'd agree with most. It sounds like the CDI unit. You're getting fuel through and have compression. It may be weak mixture I suppose, but these little engines do backfire now and then when you are trying to start them so I wouldn't assume that's it.

Is it like this?

 

[jacaldo]

It looks exactly the same as the one in your picture.

 

[Lakesailor]

Does it have that little rubber pipe that fits on the air inlet fitted? I left that off once and it wouldn't start until I put it back on. I didn't bother to investigate any further.
It really shouldn't have made any difference. Perhaps it was coincidence.

I did have a similar issue at one point. Under the internal fuel tank there is a fuel filter. It's impossible to see unless you take the tank off. It is also very small so getting a generic one to fit is tricky, but the Tohatsu/Mercury/Mariner one is about £25. I know you said the fuel flows OK, but I thought mine did as well. Renewing the filter made a world of difference.

 

[jacaldo]

I don't believe it is a fuel problem, as I said in the original post, squirting brake cleaner directly into the carb makes no difference, but every day is a school day.

 

[jacaldo]

Can't see any loose pipe, but it does have that little inlet on to of the carb

 

[Lakesailor]

I don't know what brake cleaner contains. A little bit of petrol chucked at the carb should get it to run briefly.

 

[jacaldo]

Brake cleaner is highly inflammable/flammable? (don't know which is the correct word) works just like petrol with a short burst

 

[VicS]

Brake cleaner is highly inflammable/flammable? (don't know which is the correct word) works just like petrol with a short burst

Flammable or inflammable mean the same oddly enough. Inflammable possibly the more commonly used except in technical context where flammable perhaps more widely used.

OK so that pretty much points to an ignition problem.

Easy to eliminate the plug by replacing it. Inspect the HT lead.

If it never fired at all one would check the stop button/ killcord for correct operation

But apart from that there is a LT coil under the flywheel, a pulser coil, HT ignition coil and the CD unit. Prices ranging from about $40 US for an LT coil to over $250 US for the CD unit.

It needs specialist equipment to check out a CDI system although it is fairly simple to make a dva peak voltage adapter for a digital multimeter if you are that way inclined.

 

[david_bagshaw]

if it hasnt been used fr years a little oil through the plug hole to wet the rings, improves compression a lot and will just give you the edge on starting.