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Old 09 December 2022, 11:49   #1
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Mercury 50 Intermittent Spark. Help!

Hi, I have a 1978 Mercury 50 4 Cyl 2 stroke, (I believe to be a "Thunderbolt")

However I have had consistent issues of inconsistent spark on The top 2 cylinders
(Usually the top two are the ones causing trouble but occasionally the bottom two play up as well.)

I started by replacing the Spark plugs and confirming the coils were operating correctly.
That changed nothing, and I was getting Surges of electricity Through the wiring when the engine would start, I changed the Stator and the control box
(As I thought the Stator might have fried the control box with the surges)
and also repaired and rewound the coils on the Trigger.

Its important to note that throughout the work that was being done it was used on the water (I think only running on 2 Cylinders as it was very slow, hard to start and bad on fuel)

Now the boat is being dry stored I have attempted to get to the bottom of this issue with my Mum's Boyfriend who restores Classic road vehicles, however we are both stumped with the spark issue.

It seems that sometimes when you try to start it, it fires on 2 then might fire on 4 Then no spark then very weak spark on 1. And the problem seems impossible to chase as it constantly changes where it will spark.

Does anyone have any suggestions on what the issue could be?
Or any suggestions on alternative Ignition systems I could replace the OEM System with?

Thanks in advance!

-Sam
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Old 09 December 2022, 12:36   #2
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What manual are you using to decide which components are working correctly?

You say:
Quote:
Originally Posted by SamFoley7 View Post
and also repaired and rewound the coils on the Trigger.
-Sam
What was wrong with it & are you sure that it is now working properly?
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Old 09 December 2022, 13:17   #3
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Have a look on Google for CDI trouble shooting guide
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Old 09 December 2022, 15:07   #4
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inspect all the wiring very carefully. the wiring on those older Mercs had a tendency to break down internally and give no end of problems, that has certainly been my experience anyway.
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Old 09 December 2022, 17:01   #5
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Welcome to RIB.net

I too would like to know how you determined it is spark? Are you using an automotive type scope? Primary or secondary on the scope? Or both? Got a picture of the patterns? A great way to see if the secondary high voltage wires are breaking down is to use a "Water Scope" which is just a spray bottle filled with water and sprayed on the wires with the engine running normally. If it starts to misfire then there is most likely a breakdown of the wires and carbon tracking.

What are "surges of electricity", and where is the measurement being taken? Are you measuring voltage and if so what is the result?

Have you ruled out fuel? Spraying a carb cleaner or starting fluid in while the engine is misfiring makes a difference?

Pictures of the power head with the engine cover removed would be nice.
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Old 12 December 2022, 12:17   #6
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Thank you all for your replies, you will have to forgive me as I am working with limited knowledge of Engines and outboards.
Since posting the original post the engine is now producing no spark whatsoever and now I am trying to figure out that problem.

I have measured the output of the trigger coils and they are both 29 Volts.
The stator coils read 24 Volts on the red coil and 29 volts on the blue (with the engine cranking)

It seems there is no output on the CDi module now.
I am wondering if maybe the voltage on the coils is to high and damaged the CDi unit.
Does anyone have any idea on what the coil voltage should be?

To answer your question on "surges of electricity" that was measuring the coils on the OEM Stator, One coil was 25 Volts and the other coil would continually rise while the engine was running, highest it read was 350 volts. That is when I replaced the stator and control box as I feared that the high voltage had damaged components.
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Old 12 December 2022, 12:20   #7
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I am currently not using a manual as I couldn't find one with useful information.
The problems with the trigger were; 1 coil was corroded and split open due to salt water
and the plastic body had broken in two pieces so we rewound 1 coil and glued the body back together.
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Old 12 December 2022, 12:21   #8
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It seems as all the wires are in good condition, and most have been replaced recently with the new parts.
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Old 12 December 2022, 12:35   #9
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Photos of wiring

Wiring photos as requested
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Old 12 December 2022, 14:31   #10
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Quote:
Originally Posted by SamFoley7 View Post
It seems as all the wires are in good condition, and most have been replaced recently with the new parts.
Has it ever run perfectly since you had it or has it always been like this?
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Old 12 December 2022, 14:35   #11
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Quote:
Originally Posted by SamFoley7 View Post
I am currently not using a manual as I couldn't find one with useful information.
The problems with the trigger were; 1 coil was corroded and split open due to salt water
and the plastic body had broken in two pieces so we rewound 1 coil and glued the body back together.
Seems to me you've replaced a lot of items but not the one which was obviously in bad condition.
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Old 12 December 2022, 21:28   #12
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First rule of outboard's wiring, don't trust the wiring. Get yourself an inexpensive multimeter. Manufacturers don't use tinned wiring, so if any doubt then wiring that's oxidised loses its conductivity or you have a break, otherwise you'll chase your tail on this.

Most spark issues if you know you have a solid 12v battery then HT leads, CD box wizardry, coils and stator on the flywheel. Obviously check the spark-plug condition too.

Let us know how you get on.
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Old 13 December 2022, 12:52   #13
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Do not repair the Trigger, replace it with a known good one then see how it goes.
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Old 13 December 2022, 23:12   #14
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Is this a 24volt system?

Do all the coils have the same ohm readings? Personally I never would have rewound one, but maybe they are impossible to locate? I'd be worried about carbon tracking that is not visible. There must be replacements available somewhere though.

Time to start testing components which gets harder without a lab scope. Being able to see the waveforms pinpoints breakdowns. There comes a point you replace questionable parts, then start guessing and replacing untestable items like the CDI box.

The motor and wiring are cleaner than I expected
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