Mercury Optimax Models 135, 150, Direct Fuel Injection., Page 256Get this manual
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6Chatter Marks: Chatter marks are the result of combination of low speedlow loadcold water temperature operation, aggravated by inadequate lubrication andor improper fuelUnder these conditions, the crankshaft journal is hammered by the connecting rodAs ignition occurs in the cylinder, the piston pushes the connecting rod with tremendous force, and this force is transferred to the connecting rod journalSince there is little or no load on the crankshaft, it bounces away from the connecting rodThe crankshaft then remains immobile for split second until the piston travel causes the connecting rod to catch up to the waiting crankshaft journal, then hammers itThe repetition of this action causes rough bearing surface(s) which resembles tiny washboardIn some instances, the connecting rod crank pin bore becomes highly polishedDuring operation, the engine will emit "whirr" andor "chirp" sound when it is accelerated rapidly from idle speed to approximately 1500 RPM, then quickly returned to idleIf the preceding conditions are found, replace both the crankshaft and connecting rod(s)