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Compression on an 8HP mercury

K

Koelio

New member
I tested the compression for my Mercury 8HP four stroke with the guy from whom I bought it and it gave us a reading of 120PSI on both cylinders.
When I looked it up later it showed 120 is quite low for a Mercury but I remembered the throttle was in the idle position.

Would W.O.T. give me a different (higher) reading? If so, can I assume that 120PSI with throttle 'closed' is sufficient enough?

The motor in question: Mercury 8HP four stroke - 2006

Thanks in advance!
 
Sometimes, with four strokes the throttle nees to be wide open. On small motors like that, the gage can trick you if it's an automotive designed for large volumes.

Jeff
 
Hi Jeff, thanks for your reply. Can I assume the reading would be higher with the throttle open? Or is that a wrong assumption?
I will check once I get my hand on a tester but for now I just want some assurance I didn't buy a possible boat anchor.
 
Air in is air out and the engine is basically an air pump. However, idle RPMs are up around 700 but cranking speed is around 200. So figuring the air volume at those two numbers it tells me if the engine is sucking enough air or fuel air mix to operate the engine on its own HP at idle speed, with the throttle closed, surely with the throttle closed it can suck enough air to satisfy 200 RPM cranking speed.
 
racerone said:
Throttle needs to be open.----And see if this motor has compression relief.
Click to expand...

I know that is the rule and I do that when I remember but still the thought I posted just came to me when I replied. Over the years I have usually done it with the butterflies closed on engines due to negligence.
 
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