94 Evinrude 129 hp Compression test all 4 90 psi . Installed new plugs, was running rough. How bad is 90 psi?

Other answer:

90 is very low, and probably the root cause of rough running. The fact that all cylinders are low may indicate a problem with carbon build-up in the ring grooves, which will lower the compression. I would suggest you try using a carbon removing agent. The only 2 readily available, that actually work, are Yamaha Ring Free +, available at any Yamaha powersports dealer, or Techron, which is usually available at any Chevron dealer.

Remove the spark plugs, load up a spray bottle with Ring Free or Techron, and spray an ounce or so in each cylinder. Let the outboard sit overnight. Mix the remainder of the Ring Free or Techron with your gasoline ( about 1.5 ounces per gallon). Before attempting to start the outboard, crank the engine (with the spark plugs out) for about 10 seconds to clear the cylinders. Replace the plugs, and go for a boat ride, running the engine as you normally would.

90 psi is low but even it should still ok is it a 2 cycle engine also try the ring free suggested when storing
your engine for the winter put sum oil in cylinders to stop rings from rusting good luck
Shaun H-A:
90 PSI is barely enough to combust gasoline.
Do a leak down test on it instead of a cranking compression test.


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