Mercury Mariner Outboard 225 3 Litre Service Manual 1994, Page 23Get this manual
Detonation usuany can be prevented if: 1The engine is correctly set up2Diligent maintenance is applied to combat the detonation causes Compression Check
12345Remove spark plugsInstall compression gauge in spark plug holeHold throttle plates at WO .TCrank engine thru at least compression strokes te obtain highest possible readingNormal compression is approximately 110120 psi (758.5827.4 kPa)Check and record compressior of each cylinderVariation of more than 15 psi(103.L kPa) between cylinders indicates that lower compres sion cyli nder is defectiveworn or sticking piston andor scored piston and cylinderAn engine with low or uneven compression cannot bE tuned to give peak performanceImproper compres sion MUST be corrected before proceeding with en gine tuneupCylinder scoring: If powerhead shows any indication o' overheating, such as discolored or scorched paint, reo move cylinder heads and visually inspect cylinders scoring or other damage as outlined in SECTI ONPOWERHEAD"