"I have a 1984 Egg Harbor 33' with original twin 454 inboards (1300 hours each) coupled to Borg-Warner 1.91:1 reduction gears. The port engine (which is right hand rotation with a gear driven cam shaft) has a moderate knocking noise similar to a “diesel knock”. The noise has been present since I have owed it (3 years) with 200 hours of operation and no change. It is present both in gear and out of gear and is most noticeable at idle and low RPMS. The noise is more of a knock than a tapping and has proved hard to isolate. Its does not appear to be coming from the lifters or push rods--the rocker covers were removed and it did not appear to be coming from the lifter valley.
At engine RPM’s increase the relative level of the knock decreases as other engine machinery noise drowns it out. It does not reveal itself if the throttle is increased and then suddenly lowered like a rod knock would. Ignition timing was verified with inductive light at 12 degrees BTDC. Distributor and wires are new and advance is working; ignition system is solid state Davis Unified Ignition. Fuel is fresh stabilized 87 octane with water separators and appears good. Same fuel produces no noise from the other engine. Belts from all accessories were removed with no change in noise level. Plugs are new. Engine starts right up (better than the starboard engine, has good power, and generally runs very well). Carbs were rebuilt and swapped with no change. Fuel lines were also swapped to no effect. Oil pressure is steady at 40 PSI at idle 700 RPM and 65 at 3000 RPMS. I haven’t run it hard because of the noise but this spring I’m going to turn it up and see what happens. I realize a remanufactured long block may be in my future.
Oil is clean 20W-50. A quart of Lucas Oil treatment had no noticeable effect. One local engine guy listened to it and thought it might be piston slap. He did not think it was rod bearings as the noise decreased with increased engine speed. I haven't noticed much improvement after its fully warmed up, though. I’m thinking a compression check may be in order and I may try to determine whether it occurs with every rotation of the crankshaft or ever other rotation, i.e. is it top end or bottom end noise. I replaced the drive damper but the old one didn’t seem to have any defects.
Cam gear? Piston Slap? Carbon build up on piston crown? Main bearing? Connecting rod bearing? Wrist pin? What do the experts say?"