Compression Test, Now what? 1990 Mercury 115hp Outboard

I thought i had a timing issue, and i started with a compression test.
Here were my results:
1-30
2-112
3-115
4-90

Obviously 1 and 4 are looking off, one is real scary.

This is my first OB so im not 100% s=ure what my next step is.
Should i try a can of seafoam? if yes are there any special instructions?

Is the compression too low in #1 to be a stuck ring?

Could it be a head gasket?

How do i inspect the head gasket or piston/cylinder wall?

Any hep would be great.
thanks!

Re: Compression Test, Now what? 1990 Mercury 115hp Outboard

the only way to really tell what the cylinder walls look like is to pull the head cover that the spark plugs mount into. a replcement gasket for that is about 20.00. click here for an exploded view and parts. I would be only taking a guess but if you have only 30 pounds on cylinder one, I would be concerned about a lean fuel condition that caused that on cylinder one. If the nunber one cylinder's carb has something restricing its fuel flow it could cause a detonation of cylinder number ones piston. With a lean fuel condition the top of the piston gets hot enough to start coming apart and those metal fragments grind between the piston and cylinder walls creating grooves. That low of compression can be the result of something like this occurring with your motor. Best thing to do is probably pull the head cover to see what the cylinder walls and pistons look like before proceeding.

Re: Compression Test, Now what? 1990 Mercury 115hp Outboard

it wouldn't post my link but look up the parts for you motor in any of the parts sites for mercury motors and it should show an exploded view. sorry did my best.

Re: Compression Test, Now what? 1990 Mercury 115hp Outboard

The crowleymarine web site will have parts list for that motor.

Re: Compression Test, Now what? 1990 Mercury 115hp Outboard

Looks like the head is sealed..I can't get at the pistons from that side as the cylinders are 1 whole unit to the plugs.
Does that mean I have to take the whole block apart in order to inspect?

Re: Compression Test, Now what? 1990 Mercury 115hp Outboard

Seafoam won't help you if you only have 30lbs...
Sounds like an overheat on the top piston probably did the damage.
Bottom is probably due to a bad crank seal.
In any case it's going to have to come apart and get new rings and an hone at minimum.
1 and 4 might need new pistons and a rebore as well.