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40M 4STR electrical

V

Vanuatu Pete

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I have inherited a 40 horse carburetted 4 stroke that visually appears in great condition but has a fault. Runs on 2 cylinders only runs at full revs. Merc dealer seems more interested in selling a new engine instead of fixing this one. So far ECU has been replaced as well as the coil assembly under the flywheel. Coils test fine as do HT leads and plugs. Connected to the green common wire on the coil circuit where it returns to the ECU there is a black wire that is grounded to the block. This wire appears as if it might contain a diode as it show open circuit to ground but about 4 meg ohms resistance from ground to the harness. Beats me what it is for but with it by-passed and grounded no spark and if completely disconnected the engine still runs on 2 but idles only and won't rev. Any ideas where to look before I start tearing down carburettors?
 
Simple things first....
Start with a compression check (compression over 120 psi, and with in 10% of each other),
then a spark tester (7/16" spark jump on all cylinders),

Get motor schematic (you now know what the wires connect to and can figure out what they do...)

Then go to CDI troubleshooting ...... (get DVA and Meter, read resistances and voltages) ...

By this time you have eliminated all the things you have thrown money at and are as smart as the marina mechanic....

Carb issues are typically not cylinder specific.

Check with the local marina on the cost to have the carbs cleaned by them.
Don't ever under estimate the VALUE of an hour or two of shop time by a trained marine mechanic.

Part, materials and equipment to do the job correctly are expensive.
Unless you plan on cleaning many carbs in your life spend the money at the marina.

Buy a repair manual, read, watch YouTube videos, research on the internet......

You will probably have to clean on at a time .....
Buy ALL the parts, materials, and equipment before you start....

You are probably, now, ready to start the cleaning process..... Take your time.... It will probably take you a couple days.

Let us know how you are progressing.
 
Thanks, I'll check into the compressions and spark jump. Still keen to find out what the diode or whatever it is contributes as it definitely changes the symptoms and the readings are not what I would expect from a properly functioning diode.
 
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