TOPIC: Merc 400 No Spark, Low Compression

Inherited a Merc 400 with the recent purchase of my Redfish. Compression on all four cyl is around 90. Nice that they are all the same but, this seems low to me. I have some ring conditioner soaking them in hope that they are stuck and will free up. Can anyone tell me what is typical compression for these motors? Maybe 90 is what I should expect. It is not published in the service manual anywhere that I can find.
Also, no spark on any of the 4 plug wires. Wondering what is the most common cause of that on this specific model..

Already into this thing for $45 to have the starter rebuilt. It would not spin the motor with plugs in the cylinders. Burnt brushes according to the service place.

Any help and insight appreciated.

compression 120lbs
dirty points and bad condednsor

Thanks Dave.

I have a ways to go to get to 120 on the compression. It should run at 90 if I can get spark and fuel. The cleaner will work better if I can get the motor running and up to temp and inject it through the carb into a hot running motor. At least that's what the directions on the can want me to do. I am using an OMC product a friend gave me. Have you had any experience with these cleaners preferring one brand over another for freeing up rings?
I'll throw plugs, points and condenser/s at it. Maybe I'll get lucky. I am due for some luck with these old outboards. Even if the parts don't fix this one, I can use them on another Merc down the road.

just clean the points and see if you get spark. no need to put it back in the motor. just ground the plug to the mag and spin by hand.

Like Dave said, clean the points. New points are Expensive for these motors. Of my 5, I have new points in one only. A good clean and polish will work like new.

Yuppp,...My bet is that polishing the points will revive the spark.
Set the points at .008" once they are all polished up.
The OMC 'engine tuner'?? designed to soften carbon is a good product.
I prefer CMC engine fogging spray.
While you have the magneto off, or be sure and attach a ground wire to the kill terminal on the port side of the magneto,...I tilt the motor up to where the sparkplug holes are as straight up in the air as possible, then slowly rotate the flywheel as you spray in the product of choice. Leave the beast in this position, noting which pistons are up and which are down. Spray some extra in on top of the two pistons that are up.
In a couple days rotate the flywheel slowly a few revolutions as you spray in more product, and then leave the beast sit like this with the other two pistons up. Spray some extra in on top of the two pistons that are up.
In a couple days purge the cylinders with fuel mix as you slowly rotate the flywheel about 50 revolutions.
Then tilt the beast back up-right, and place a drain pan or rags under the exhaust dump and let the excess fuel mix and carbon softener ooze its way out for a couple days.
Then clean up and do new compression tests.
I'm betting that your 90 psi tests were done without any pre-lubing??
Be sure and use a compression tester that screws into the sparkplug hole, and spin the motor with the electric starter until the compression gauge stops climbing.
Other carbon softeners that work include Marvel Mystery Oil, and a 1:1 mix of acetone and ATF.
If your compressions don't come up after these attempts, remove the starboard side transfer ports covers and see if there is any visible scoring to the sides of the pistons.
Even though there is a much higher likelihood of scoring on the port sides of the pistons, please don't attempt to remove the port side water & exhaust cover to check for scoring until you ask for someone to post a few proven ways to remove the port side cap-screws without breaking them off.
I may have put info in this post that is already something you know, but I include everything I can think of just in case.
doc

Just to add to the Dr.'s good advice, try another gauge to be sure, they never all read the same it seems. If the numbers you are getting are all even then chances are the engine is probably fine, typically the readings will vary with a bad engine. To be absolutely sure a cylinder leakdown test will tell all, regardless of what the compression gauge reads. I always like to do both to be sure.

As far as the mag goes, they are simple to repair but if it is your first one, let us know and we will walk you through the teardown and repair, the caps and rotors are easily damaged if treated roughly, and cost $$$$, so be careful. Oh yes, best to get a factory manual and read it cover to cover. Money well spent.