Compression Question

[roystenstrom]

I purchased a small 2001 flats boat with a 2001 90hp mercury salt edition. It runs great on the water and has recently had a full service in March.

I’m not a boat mechanic. However, considering the age of the motor, I recorded a compression reading of 74, 74, and 71. My process was to pull the kill switch and test each chamber, pulling out one spark plug at a time.

I don’t know the hours, however can I assume that if three chambers of a new engine are to test in the 100s that this engine to test in the mid 70’s (within 10% of each other) Is just normal wear on an 18 year old engine?

Hope this makes sense,
Roy

[JOE54]

tested wrong , likely wouldn't even start with 70 lbs or run very bad , warm it up pull all plugs grnd plug wires and recheck with throttle wide open and hot battery

[roystenstrom]

I believe we tested it cold without warming it up... but sounds like I need to try it again like you described

[JOE54]

[lpugh]

^^^^^^^^^^^^^
TIP: useing tire valve cores in a compression gauge will usually cause the gauge to read low by quite a bit, check the gauge accuracy
Your reading have very little variation which is great, I doubt you really have low compression

[JoshKeller]

Are you by chance using a harbor freight gauge with the screw in hose?