Low compression results... would you trust this gauge?

I have 94 Merc 25hp 2-stroke that I've been running for about a year and a half. Last year, compression numbers were "real good" according to my mechanic. Don't recall what he said they were though.

Last weekend while out on the water, it started fine, idled fine, etc. but would not accelerate or rev beyond 25% or so when I opened the throttle. I checked the linkage, etc. to be sure the throttle was in fact opening up, and it was. I got it hope, checked the sparks and the plugs themselves, no issues. The motor would rev with no load (out of the water).

I borrowed a compression tester and got 60 & 60 on both cylinders. This is ridiculously bad compared to the performance I was getting, and results from a year ago or so. Hard to believe it good go that far south in such a short time without doing something drastic like not putting oil in the fuel. I do, btw, and in the right ratio.

Since both cylinders are bad, I'm assuming it has to be something affecting them both. I pulled the cylinder head cover on this motor, but the one on the back just covers the tops of the cylinders. Can't see inside other than through the spark plug holes. It's really the cover for the reservoir the water flows through on the way out for cooling. I don't see how this could affect the compression inside the cylinders. Trying to find if there's another gasket deep down in there. Since it's both cylinders at the same time, decreasing by the same amount, I doubt that it's piston rings, pitting/scarring on cylinders walls, etc.

Which brings me back to questioning the comp test to begin with.

Some of the mercury 25's were known for cylinder plating delaminating, which will obviously cause a sudden loss of compression. I'm not familiar with Merc much, so this is something you might google or ask about. I think the plating is called "Mercasil" or something along those lines.

Thanks for the heads up - I went and googled it and it doesn't look good.
Hopefully that's not it... or it's time for a new motor.

May not help, but from my hot rod days, we used to squirt oil down the cylinders via the spark plug holes and if the compression readings increased, it usually meant some bad rings/cylinders.

Well, yesterday I went at it with another gauge, and lo and behold I got 115 in both cylinders
So that's good. Or much better, anyway. Still lower than service manual says I think it was around 120.
Now i don't trust the gauges. Nothing worse than a faulty gauge, can lead to a whole lot of unnecessary work.

Wondered, after your thread headline, if you'd only used ONE guage.

Maybe...you might try yet another guage...sort of a tie-breaker?

Then, depending on which result is ratified, you can start the search for the problem and solve it with.


Best wishes.

[url=https://www.tinboats.net/forum/viewtopic.php?p=324439#p324439 said:

Kismet » 27 minutes ago[/url]"]Wondered, after your thread headline, if you'd only used ONE guage.

Maybe...you might try yet another guage...sort of a tie-breaker?

Then, depending on which result is ratified, you can start the search for the problem and solve it with.


Best wishes.

Click to expand...

Yep. Already borrowing a 3rd to be sure.
I think the first gauge had a faulty connection between the hose and threads that go into the plug holes.

Now I still have the original problem.... but whatever it is it just got potentially cheaper I think.
Since it was only going to 25% max rev on an open throttle under load, I'm thinking it was on 1 cylinder. I'll test that tonight. After thinking the block might be damaged, spark issues don't scare me so much.

Gauges can vary as you found out. I have three gauges for my compressor and tires. They all vary on the same pressure source.

Can't give too much advise on two stroke motors but going back to my hot rod gear head days, I once had an engine that idled well and ran well at low speeds but as soon as I got on the highway, it lagged tremendously. It turned out to be bad crankshaft bearings. Timing is also a critical item also.

[url=https://www.tinboats.net/forum/viewtopic.php?p=324451#p324451 said:

MRichardson » Today, 10:14[/url]"]

[url=https://www.tinboats.net/forum/viewtopic.php?p=324439#p324439 said:

Kismet » 27 minutes ago[/url]"]Wondered, after your thread headline, if you'd only used ONE guage.

Maybe...you might try yet another guage...sort of a tie-breaker?

Then, depending on which result is ratified, you can start the search for the problem and solve it with.


Best wishes.

Click to expand...

Yep. Already borrowing a 3rd to be sure.
I think the first gauge had a faulty connection between the hose and threads that go into the plug holes.

Now I still have the original problem.... but whatever it is it just got potentially cheaper I think.
Since it was only going to 25% max rev on an open throttle under load, I'm thinking it was on 1 cylinder. I'll test that tonight. After thinking the block might be damaged, spark issues don't scare me so much.

Click to expand...

Yep. I think you have an easily solveable problem. Congratulations. Hope the solution is simple.

I had what sounds like a similar problem with my 15 HP Johnson last year. I took it out and it would only run to about 1/4 of its normal power (less speed than the 5 HP I had on the boat previously. Problem went away when I gave the carb a good cleaning. the carb didn't appear to have any gum or varnish in it either, but there was definitely something in there that came out. :mrgreen: . Maybe your problem is just as simple. Best of luck with it.

Just had a similar problem with my 9.9 johnson. Took it in to get fixed and they replaced the points and contacts and now it runs awesome.