Why won't my outboard start?

[mark_turner]

Why won\'t my outboard start?

Two weekends ago I was motoring up the Medina after a nice sail from Warsash to Cowes when my 1997 4hp 2 stroke Merc stopped. And wouldn't restart. After sailing up to the pontoon at the Folly in I cleaned the carb and replaced the plug with a new one and it started again and ran fine. So I had some lunch before restarting it ready to cast off and head back home. But it stopped again and has not run since.

At home I had a good look and it appears that fuel system is fine but the spark is VERY weak, intermittent and does not appear to be timing correctly.

I would like to work out what is wrong with it without resorting to taking it in to be looked at but how can I diagnose the fault at home? There are only a very few components to the ignition system but I don't know how to check which one is broken.

I hope someone might be able to advise.

Mark

 

[charles_reed]

Re: Why won\'t my outboard start?

I think yours is one of the re-badged Tohatsu's.

If it is, they've got a sealed ignition system and you'll probably have to replace the whole lot if it is the unit that has gone wrong.

So before committing yourself to ££s of expenditure I'd go through the following simple checks.

1. Try with a new spark-plug - the insulator on the one you have may be tracking down the outside. If that doesn't clear the problem
2. Change the rubber cap over the spark-plug and renew the HT lead.

If neither if those actions clear it up, bite the bullet and get a new ignition unit - but from a Tohatsu NOT a Mercury dealer.

If I'm wrong and it's not one of the Tohatsu's and it hasn't got a sealed ignition unit then your problem is simply remedied if somewhat of a pig to carry out - you need to change the points and condenser. To do that you'll have to pull the flywheel off.

 

[spannerman]

Re: Why won\'t my outboard start?

If its a '97 model it won't have points and a condensor as most manufacturers dropped this system years ago. If the spark is weak as you say, try with a plug opened up to about 5mm gap it should fire easily across this with a lilac coloured spark, if it is yellowish and weak then you could have a problem with either the stator or trigger unit (both under the flywheel which generates electricity for the ignition system) or the switch box (ignition module). You can test the stator with an ohm meter should be just two or three leads down to the switch box, two of them are from the stator and the third from the trigger, I don't know the colours and readings of the top of my head but can find out. If the spark is good but erratic its probably the trigger, if its weak the stator isn't making enough current.
I have changed dozens of Mercury stators that ran for a while but when they get warm they break down or run erratically. Switch box's usually work or are totally dead.

 

[grinch]

Re: Why won\'t my outboard start?

The bigger mercs were fitted with a safety tilt switch which is prone to fail. If your engine has one, try disconnecting the black/yellow lead which goes to earth and then fire it up. The stop button can also fail by providing a partial earth. same remedy as tiltswitch.

HTH

Grinch

 

[clyst]

Re: Why won\'t my outboard start?

Spark plug gap of 5mm ???? Thats nearly 1 /4in!! Error ol' chap??

Regards

Terry

 

[tr7v8]

Re: Why won\'t my outboard start?

No it's not, in open air at STP 5 mm is about 5KV and any half decent coil system should fire this.
From distant memory it's about 1KV for every 1 millimetre of gap.
The reason you run smaller plug gaps normally is that the combustion pressure means higher volts to jump a given gap.

JIm
------

 

[sonarbell]

Tr7V8

I hope you dont run your Tr7V8 to the revs produced on your rev counter. Anyway, I think you are about right in your assumptions regarding 1mm per KV in open air. And definitely right regarding the relationship between combustion pressure and Gap.

 

[spannerman]

Re: Why won\'t my outboard start?

I seem to have generated a whole new subject here on spark plug gaps, glad to see that a few of you understand what I am getting at, the proper workshop ignition tester we use has an adjustable gap from about 4mm to 10mm, and a correctly operating outboard system will easily fire this gap. Reason being unlike car systems which run on 12v an outboard uses the stator to charge up a condensor which then dumps about 300-400v into the coil, so you get a very strong spark. Hence all the warnings in the manuals about water, and heart conditions etc, as these can really give you a belt.
Going back to the problem, if you still have it, you can test the following, first disconnecting the leads at the bullet connectors;
Stator between white and red/ black leads 93 -142 ohms
Trigger " " white/red and earth (black lead) 80 -115 ohms
Coil primary circuit between two spade terminals .020 - .038 ohms
Coil secondary " " between one spade terminal and plug lead 3000-4000 ohms.

Run the engine till it gets warm and the fault re-occurs ideal if it stops as then you can see which component has failed. if they are all OK and you are sure its not a fuel problem then it only leaves the Switch Box, and its impossible to test these without special test rigs. So its a process of elimination. Good Luck Steve

 

[AOWYN]

Re: Why won\'t my outboard start?

In the USA gasoline has had 10% ethanol for some time. Local outboard specialists are suggesting problems with fuel deterioration in as little as two weeks.
I would recommend complete change of fuel to fresh stuff.

In the meantime I am hoping that a new spark plug will fix a non-working Mercury 4hp motor later today here in sunny Florida.........