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Thoughts on Compression Readings on 2000 Mercury 90hp 4-Stroke

thompsdw

New member
After a minute runtime:
#1: 135
#2: 120
#3: 140
#4: 110

After 5 minutes runtime:
#1: Didn't check
#2: 100
#3: 132
#4: 75

I expected to see the compression come up on the readings after the engine was warm. The engine is amazingly clean and runs very well at speed and idle. I believe factory spec on compression is 135. I am a little worried about #4?
 
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What I am trying to determine is if carbon can be playing a role in the readings above. I am suspicious that is the case. From what I understand, this is a Yamaha powerhead and they tend to generate carbon. While I know anything is possible, the engine is very well maintained and the initial readings were pretty good. I am surprised the readings go down as the rings should swell, but maybe if they are locked in carbon the cylinder is expanding when hotter and the compression drops. The valves would have problems at any temperature I would think.
 
On a 4 stroke outboard you need to run a leakdown test as this will give you ring condition and if valves are sealing..
 
I can do that. The outboard is not mounted right now, so I'm thinking about orienting the pistons upward and then pouring something into the cylinders to soak into the rings. Any suggestions on what to use? Any issues with doing this?
 
4stroke rings do not carbon up like 2 stroke engines, only way they would be sticking would be rust in cylinders (needs repaired) or cylinders scuffed from lack of oil or overheat.
 
Sticking valves? Run a mix of 20% Seafoam and 80% clean engine oil for 20-30 mins. at 1500 RPM. Change oil and filter and retest.
 
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