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Help with 25hp 2 stroke Mercury 2000 yr model

R

Rett

New member
New to this forum. Out of ideas.

Our mercury 25hp has been cared for since new. Last outing, it began losing one cylinder (loss of power), and would recover if we mashed on primer bulb constantly (got us home).
Carb was cleaned and inspected (dirty but nothing suspect).
Reassembled, engine will only fire intermittently now, Appears to get spark (HF tools spark tester, and grounded sparkplug method) to both plugs (new NK plugs)
With the carb off, I looked at the reed valves and they looked clean, closed and would return to position when I put pressure on them.

Fuel makes it to bowl (inspected fuel pump on carb when carb cleaned) and I can quirt fuel in engine with blue primer button on carb is pressed (also suggesting bowl is full). Occasionally will fire when I do this.

Compression (cheapo compression gauge measures 20psi on both cylinders (very low). However, I have a hard time believing that both cylinders have failed (unless head gasket failed...(which I see no sign of) given that this motor was pushing 4 people around last outing.

Not sure what to explore next.

Thanks in advance,
Rett
 
Borrow a decent compression tester (AutoZone) and see what's really going on. That 20 lbs is NO compression!

Jeff
 
Thanks for the quick response.
I tested the spark from both coils and it easily jumps a 3/8 in gap.
I tested my compression gauge with my air compressor set regulated to 60psi and held air to the end of the gauge...reads abt 60psi. This leads me to believe that it is accurate and that I have no compression on either cylinder.

I noticed today what I perceive to be an excess amount of oil (brown residue) in and stuck to the sides of the water tank housing the lower unit. Hard to believe that this much oil got there from test cranking (by hand) alone (and past cylinder rings), unless oil/gas was getting directly into water jacket. This + compression issue suggests head gasket to me.

My hesitation is that 1) the gasket had to break inside (there is no sign of leaks from outside) in such a way to effect both cylinders...but on this particular head gasket the cylinders don't share a common section which could break out across to lose pressure on both.

My questions are:

Anything else to test before pulling head? and
Could the exhaust manifold gasket failure cause decreased compression? (do I need to look at this one?)...I'd rather not go into the exhaust manifold side if not necessary...just 9 extra bolts that could stick / break if I'm not careful.

For the head gasket should I use a copper gasket spray (permatex) or leave it dry (pressure only and torque it down properly).

Thanks.
 
The engine does not have a head gasket. It is a blind bore engine. The bolts that appear to be head bolts are only retaining the block cover plate, which removing them will expose the cooling area that surrounds the bores.
 
Good to know. Thanks. Not sure where to go from here.
I don't really want to pull this engine apart if I don't have to...
Still can't imagine a scenario where both cylinders compression are compromised given the spatial distance between the bores.

Hopefully I'll see something with the cover off. Open to any other ideas...
 
Given that there is no head to pull (cover only gives access to water jacket)...how would I (without a scope) assess the shape of the cylinders/rings?

I've read elsewhere that an exhaust gasket leak could allow moisture into exhaust port causing cylinder 2 combustion to fail...and that this is a common mercosil engine problem?

I can pull the exhaust manifold to see if I can spot leakage, but I'd rather see the condition of the cylinders given the low psi of the cylinders. Difficult to get to on these mercs?

Just need some suggestions on next step. Starting to worry that this powerhead is toast.
 
There are covers on the side of the block on many of these Mercury engines.----Remove them to inspect pistons / rings.-----" cared for " means that it had a new impeller about every 5 years or so ?
 
On the throttle side of the block are 2 small covers pull those off. I like to change the water pump on the mercosil blocks every 2 yrs and no more than 3.
 
Are you certain that these covers provide a view of the cylinders?
There are 4 covers on that side but seem too far to the right.
 
Since you're not familiar with this engine, go to the Mercury parts tab on this website, plug in your serial number and go to the exploded views of every part on the engine by component group and LEARN. Then come back and ask questions about what is what.
 
racerone: Not suspicious. My limited experience with outboards is that the more I remove, the better my chances of breaking a bolt. I'm still tapping, heating, and soaking 2 of 9 bolts on the exhaust manifold side.

fitz: I have looked over the exploded views. Still not clear to me. Looking for advice from someone whose dealt with these Mercosil's...not having a head makes them foreign to anything I've done before.

I've read more about Mercosils on other sites...but no videos or explanations out there on how to visibly assess cylinder damage. Guessing the pros use a camera from the spark plug hole.
 
First; Mercosil is not anything more than a unique cylinder liner material, its not anything special over regular iron sleeved engines. For all you old timers a much advanced Chevy Vega cylinder liner design which was 40 years a head of it's time but went horribly wrong because it could not be machined correctly among other things. Second, a blind bore Mercury design has been around since 1939, with the exception of V6 2 stroke engines with removable heads, just about every Mercury 2 stroke engine has blind bores. It's not special to your challenge. Once you get the exhaust cover off you will realize it was either cooked alive from overheating or run without oil or coked up with carbon so bad that the rings are stuck to the pistons either of which will get you to 20 psi compression fast.
 
The intake covers are NOT water cooled and easier to remove.-----If compression is a true 20 PSI that will tell the story.
 
fitz73222 said:
First; Mercosil is not anything more than a unique cylinder liner material, its not anything special over regular iron sleeved engines. For all you old timers a much advanced Chevy Vega cylinder liner design which was 40 years a head of it's time but went horribly wrong because it could not be machined correctly among other things. Second, a blind bore Mercury design has been around since 1939, with the exception of V6 2 stroke engines with removable heads, just about every Mercury 2 stroke engine has blind bores. It's not special to your challenge. Once you get the exhaust cover off you will realize it was either cooked alive from overheating or run without oil or coked up with carbon so bad that the rings are stuck to the pistons either of which will get you to 20 psi compression fast.
Click to expand...
Mercosil is what the whole block is made out of. That is why the Mercosil blocks can be bored it is not just piston liner. The earlier blocks were chrome plated bores.
 
Yes the Mercosil blocks can be bored but need a diamond hone and brush to finish. When I bored these the stones for my hone was nearly $150 ea...that why to costs more and not all shops can bore.
 
racerone said:
The intakegas covers are NOT water cooled and easier to remove.-----If compression is a true 20 PSI that will tell the story.
Click to expand...

Took off the intake covers and didn't see anything obvious (to me). Cylinders had patterning in them, which I took to be part of original honing process. No scarring in the direction of piston movement that I saw. Rings intact.
Finally got exhaust manifold off (see attached pic). Wrecked gaskets coming apart...but does anything look out of sorts here?
Plan is to put on new gaskets and hope that a small water leak here was causing poor firing (and that my compression readings were way off).
and..should I use permatex copper high temp sealant spray on gaskets for assembly?

25hp mercury exaust .jpg
 
Agree with Mr. Scott, your gauge is bad. Cylinder/piston evidence, looking at picture you produced, does not raise suspicion, this as well as looking at your symptoms..... you have fuel problems. Considering your primer bulb action, which brought boat up to speed, I would say fuel....somewhere between the motor and fuel tank, assuming your carb/fuel pump rebuild was well done.
 
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