Compression Test on 225

What should the compression range be for a 225 merc?
Is 160 on all cylinders too high? 160 is what came back on a boat I've been looking at.

Year and model of Merc.?

2003 pro xs

160 seems high.

warm the engine up and take another reading.

if it still comes up high... try a different gauge.

160 psi , puts ya race fuel specs.

i think it should be closer to 110 to 120

Tommy at Rockwall Marine will know for sure

160 is high. Carbon build up, modified cylinder heads? I would recheck it. Ones I have tested I think are about 120 - 130.

And use a different compression gage.

Thanks for the input guys, I thought it seemed high as well

that would make it an "opti pop"

I would have thought that if you had carbon buildup the compression would be lower due to sticky valves not closes as they should.

I have a 90 hp Mercury and my compression is supposed to be at 220 psi which I think is super high but that is how they designed the motor.

Waco if your compression was that high you wouldn't need sparkplugs. It would detonate
like a diesel. I might agree with it being 120 but hell my sons 565 cubic inch Chevy
with 14 to 1 comp. isn't but 170. and it makes 1300 horsepower.

Johnny

Mine was 119 PSI the last time I checked it.

These later model engines are looper motors. The comp. on these engines are anywhere from 95 - 120. Like it was suggested try another comp. gauge.

no "valves" on an outboard motor.

my 300 drag with o-ringed 17CC heads made 260 lbs compression.. had to run 112 octane fuel or higher.
that motor hit hard at 11k rpms.

a 90hp engine runs about same compression as the bigger ones. around 110- 120 is it.

imho. compression over 140-150 needs race gas.

The compression can be altered a lot with the camshaft on 4 strokes. Surprising Steve but those encapsulated cylinder head motors do have more compression but not over 135-140lbs. I laughed when I saw the sticky valve post too.

i didn't realize he was referring to a 4 stroke.

I'll post pictures of the compression test if you guys would like. I have taken it to 2 local boat repairs shops trying to get the fuel delivery problem fixed. They both confirmed that my specific Mercury motor is designed to operate at 220 psi compression. Look it up if you would like or check out the links below.

What's funny is that some of you old timers know less about motors than you care to admit. Time to catch up with the times guys.


https://youtu.be/fjS3CiiXOww

http://www.louisianasportsman.com/details.php?id=4989

Waco, you chimed in on a thread in which the original post was asking about compression numbers on a 225 pro xs which is a 2 stroke motor. Totally different animal than your 4 stroke motor. It threw a wrench into everybody's thinking when you posted 4stroke compression numbers and talking about sticking valves when the thread is about a 2 stroke motor. So I wouldn't get on here and tell these old timers to catch up to the times when you clearly don't know the difference in a 2 stroke and 4 stroke.

On top off all that wkh2006 said a four stroke can have cranking compression and have very little running compression. Camshafts in four strokes have intake and exhaust overlap that lowers the running compression. We do not care about people trying to jump a thread, with un-relevant information. Good Luck with your supply fuel pump problem.

this showed all your infinite knowledge of Merc/Mariner outboards immediately..even AFTER he says its a Pro XS.. most of us "old guys" blew ya off when you posted this on a thread that had ABSOLUTLY NOTHING ta do with what was being asked.....jmo

I have been doing this for 40 years, yes I'm one of the old farts, but I have never seen compression in a two stroke motor over 175 psi. This was in the "old " crossflow designed motors!