Low compression in bottom cylinder

[shane-r13]

So i have a motor its newer. 9.9hp mariner. I know its not classified as antique but there seems to be lots of knowledge on this site. Its compression is testing at 135psi top cylinder and 110psi bottom cylinder. Obviously there could be lots of reasons why but I’m curious as to what i should check first. It runs fine and i was really surprised to see these numbers. Rings? Maybe stuck ring? head gasket? No obvious signs of leakage. Crank seal? I run my motors pretty rich 40:1, so plugs are usually dark to begin with but both plugs look the same.

I just too this out of winter storage as well, so i’m hoping just a stuck ring, but i have never compression tested it before so could have been like this for a few years now.

Shane

[nj-boatbuilder57]

quote Shane.R13:

It runs fine and….

Just my opinion, others may disagree. But to my mind, "it runs fine" is the answer. I love turning a tool as much as the next gearhead, but sometimes curiosity makes us overthink these things.

Your time might be better spent out on a lake in your boat…..I’m not sure there’s a "problem to be fixed" here.

[The Boat House]

Seem "If it works – don’t fix it" would be the best advice.

[crosbyman]

use and enjoy it till the top cyl drops to 110 🙂

Joining AOMCI has priviledges 🙂

[fleetwin]

Yeah, I would certainly question your compression readings if "it runs fine"…I would take it out for a good run on the water, then recheck compression….

[outbdnut2]

I never check compression unless there is a problem with the way it runs.
Dave

[dan-in-tn]

I would give the motor a good dose of Merc Power Tune or OMC/BRP Engine Tuner. It will remove carbon from the rings & combustion chamber. Let it sit overnight, but no,longer than 24hrs.
I believe the earliest version of this motor was a chrome bore? Later it became a Merc A Sil block. The chrome bore has to be sleeved if it was damaged, but a cylinder can be cleaned with myratic acid if just some aluminum has transferred from the piston due to a slight overheat? Unfortunately no cylinder head! Complete tear down to get there for that repair. Most likely going to want to replace the piston/rings on that cylinder. The later Merc a Sil block is not so forgiving. It can be bored once .015 with Merc pistons, but Wiseco offered a .030 also. This is a very light block. Good running motor & easily carried, but not forgiving at all for slight overheat or run with improper oiling. Keep you cooling system in top notch condition!
The motor can be sleeved & we did a bunch of them. Expensive, but makes it even a better, more forgiving engine. A tad bit heavier, but oh so heartier. Engineers get into a pushing contest to see who can build the lightest motor.

Dan in TN

[shane-r13]

While yes it runs fine but i’d rather do preventative maintenance then deal with a blown motor later. No head and complete tear down does not sound fun though. I think i’ll take Dan In TN advice and put some Merc or BRP tuner in it. Can’t hurt. Plus i’ll run it a bit and re test and see if my weird numbers maybe from storage.

Thanks

[jerry-ahrens]

quote Dan in TN:

I would give the motor a good dose of Merc Power Tune or OMC/BRP Engine Tuner. It will remove carbon from the rings & combustion chamber. Let it sit overnight, but no,longer than 24hrs.
I believe the earliest version of this motor was a chrome bore? Later it became a Merc A Sil block. The chrome bore has to be sleeved if it was damaged, but a cylinder can be cleaned with myratic acid if just some aluminum has transferred from the piston due to a slight overheat? Unfortunately no cylinder head! Complete tear down to get there for that repair. Most likely going to want to replace the piston/rings on that cylinder. The later Merc a Sil block is not so forgiving. It can be bored once .015 with Merc pistons, but Wiseco offered a .030 also. This is a very light block. Good running motor & easily carried, but not forgiving at all for slight overheat or run with improper oiling. Keep you cooling system in top notch condition!
The motor can be sleeved & we did a bunch of them. Expensive, but makes it even a better, more forgiving engine. A tad bit heavier, but oh so heartier. Engineers get into a pushing contest to see who can build the lightest motor.

Dan in TN

I totally agree…. pick up a can of Quicksilver PowerTune, and use per the directions. It may be the best money you could spend on your motor. Recheck compression after a water test. Otherwise, if you have an inspection camera, you could look into the cylinder and check for damage. I use PoweTune on just about every outboard that comes through my shop.

[shane-r13]

I ordered some. See what happens.

Thanks

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