Mercury 50 Intermittent Spark. Help!

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Mercury 50 Intermittent Spark. Help!

Post by Fleeco7 »

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!

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Joined: Wed Mar 31, 2010 12:13 am
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Location: Donegal, Ireland

Re: Mercury 50 Intermittent Spark. Help!

Post by haventaclue »

My first thought was trigger.
Test the trigger: Between brown and white/black 70-100 resistance and between white and violet 70-100. Disconnect the wires from the switch-box first.
If not the trigger, it's the switch-box.
no matter how bad it is,it can always get worse,I'm an optimist

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