20 hp Mariner low compression

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Feb 3, 2011
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Golden Meadow, LA
I was given a 20 hp Mariner today, 1998 year model. I was told it has low compression and that was confirmed by pulling on the rope, barely any resistance. Where should I start first? Bad piston/pistons? Head gasket?
 
If it has bad compression it needs to come apart, you'll be looking at these items no matter what the actual problem is.
 
Sometimes outboards that have been run with too much oil in the mix will have the piston rings carbon up to the point where the rings are stuck in the grooves causing low compression.
 
On a two stroke it's normally pretty easy to get a look at the top of the piston/cylinder to know if there is a big carbon mess inside. Then you can clean and soak and stroke and and and … put a new gasket on and back together.
About the cheapest try considering how old it is.
 
Or too little oil will ruin not just the rings but the piston and cylinder.
 
Typically too much oil won't stick the rings. Low quality oil will.

Take a look inside the spark plug hole and if it is not a disaster, try a form of solvent to unstick the rings. Believe it or not, automotive spray gun, paint gun cleaner (it's a thin, strong version of paint stripper) with methylene chloride will do it quickly, but you have to really careful not to get it anywhere other than on the rings and piston, as it will destroy rubber seals and gaskets. So put the piston up towards the top when you pour a little bit in (like 1/2 ounce) . Do one piston at a time. Be ready to flush it out with gas, spirits, parts cleaner or other solvent.
 
The head gasket is about $5.00, and the head is easy to remove.

The only issue is corrosion, if the bolts are stuck they tend to break off.
 
Originally Posted by ondarvr
The head gasket is about $5.00, and the head is easy to remove.

The only issue is corrosion, if the bolts are stuck they tend to break off.

These engines do not have heads, they are a one piece cylinder/block. Have a split crankcase.
 
do you see any evidence of mud dauber nests inside the cowl. they can break off and get ingested, ruining the rings.

rod
 
With any 2-stroke (chainsaw, weedwacker, outboard, etc.) that exhibits low compression, the first thing is to look into the cylinder. If the piston and the bore are scored, it's not simple. If the piston skirt looks good through the exhaust port, it's almost certainly rings. They could be stuck or they could have been run hot and lost their temper so they don't have enough tension, or both ...

First step is to get a compression gauge/meter. Do a dry test. Say you get 20 PSI (really low) and then you squirt some motor oil in through the plug hole (any kind is OK for a test) and test compression again. Say it's 80 PSI the second time. OK, we have found an issue - ring seal.

Lay the motor down with the plug holes facing straight up. Plugs out position the pistons so they are above the ports. Get some rifle bore cleaner for carbon fouling (I like Break-Free) and put a good slug in each cylinder overnight. Come out the next day, hang the motor on the fence or tailgate and spin it over with the plugs out until it stops spitting liquid. Pour a bit of pre-mix in each cylinder, put plugs back in, and yank that starter rope. It should sputter to life for a few seconds.

Now pull it agian and see what it feels like? If you can feel compression , you are half way home. If it's still weak, the rings are shot ... After a soak and short run, it should meter out higher on the compression test. 60 PSI and above, you have a runner. Below that, you have an overhaul to do.

Little motors are not hard to overhaul. Take your time and make sure all the gaskets seal well and you'll be fine :)
 
Remember these engines have compression relief holes in the block to make for easy starting. Hard to tell the shape its in by pulling the rope. Have you had it runing?
 
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