i have a 2003 mercury 50hp. had no spark, so i unplugged both…

What your seeing is the CDI box.

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Now if the engine is running, and you ground that black/yellow to the block right there, and the engine does not shut off, the CDI is bad.

If it does shut off, then what your looking at is either a bad keyswitch, or the harness that runs from the motor to the keyswitch has a break in it.

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Post back with questions

Gotcha.

You do need an adjustable spark gap tester to see if you have good spark and not weak spark. These are the testers we use in the industry.
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http://www.stevensinstrument.com/spark_checkers.htm

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But you can also get cheap spark checkers at most good autoparts stores.

You need to hook the spark tool up and set the gap to 7/16ths of an inch. The spark should easily jump that gap, be hot and blue in color, and be consistant with engine rpms. Weak looking yellow spark will not cut it.

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When doing spark testing, you really have to use a spark gap tool. The engine needs 20-30K volts to fire the plug when running. But only needs 3 or 4K volts to make it spark as your seeing it now, grounding a plug to the block. If the spark can jump a 7/16ths gap on a spark tool, then it is makeing 20-30K volts. If it doesn't jump that gap, then its not making the voltage required to fire the plug when running.

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That is your next step here.

Your at either a bad stator or a bad CDI box.

Got it.

On this one it has what they call CDM ignition. Instead of 1 single CDI box and 3 seperate ignition coils. It incorperates the CDI and the ignition coil into the same box. .

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Now if you disconnect the black/yellow of the rev limiter only, and you get spark back, that points to a bad rev limiter.

CDI Electronics is an aftermarket company that makes Mercury ignition system components. If you see stuff labeled CDI that simply means it is an aftermarket part, Mercury does not label their parts that way.

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Now when it comes to the rev limiter and spark. Not all models have the rev limiter. So what you can do for testing purposes is completely disconnect it, all of it, and toss it on the workbench. If you still have no spark the rev limiter is not the problem. If you get it back it is the problem.

The boat is probably not going to the lake tomorrow, sorry to tell you that. But you need a spark gap tool and need to check for spark the correct way. It is a test, and it is pass/fail. If it fails it takes some special voltage meters and adaptors to find out why, and to test individual components.

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As far as that rev limiter. Again not all models have it. If you just disconnect it and take it off the engine and throw it on the workbench and the engine runs, then that is the problem. If nothing changes, that is not it.