Compression readings high - 71 Merc 800

M

Moody Blue

Captain
Joined
May 24, 2004
Messages
3,136
Performed a compression test on my 71 Merc 800 and was surprised to find such high readings. Did the test a couple times and got similar results.

#1 - 162
#2 - 173
#3 - 170
#4 - 157

As far as I know, this is an all original motor. Factory seals still on the crankcase halves. Is it typical to see such high numbers for this age of motor? What issues could it cause regarding use of regular octane gasoline?
 
daveswaves

daveswaves

Ensign
Joined
Mar 22, 2002
Messages
901
Re: Compression readings high - 71 Merc 800

Not typical but by the same token there is no way for the previous owner to raise them per se. Everything could just be loaded up with oil from storage and your numbers are just high. I would not expect any problems with the fuel as long as you dropped the timing to 21 degrees. when you did the linc and sync.
 
C

CharlieB

Vice Admiral
Joined
Apr 10, 2007
Messages
5,617
Re: Compression readings high - 71 Merc 800

I'd be suspicious of the gauge used to get those numbers.

But then again, I had an old Harley that would coke up the pistons with hard glassy carbon (coke) and raised the compression, she ran GREAT, til I cleaned it off.

Sorta wish this one would do that. (Big Sigh).
 
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