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Load Knocking

M

MSJaxJag

New member
I recently purchased my 1999 Montery 18' bow rider with a Mercruiser 3.0. I have had it out twice and both times had heard very load "knocking" (the best description I can give) when I am at about 4200RPM. I have great/steady oil pressure at all RPM's. If I back off it goes away. My speedometer is down right now so I am not sure how fast I am going. I will say it does not sound like "valves knocking".

Any thoughts of what to check out???

THANKS IN ADVANCE !!!
 
Use a broom handle to isolate the cylinder that the knock is coming from...hold the handle between the block and your cheek/ear. The noise w/transmit via the handle. The noise could be piston slap, loose wrist pin or a bearing. The engine is 13 yrs. old...run it until it blows, have it torn down and worn parts replaced or install a reman. longblock.
 
Valve rattle is typically from pre-ignition.
The knoking that you describe may be caused by one of several things as posted above.

Detonation is another possibility.
Some causes:
Too much spark lead per RPM.
Excessive cylinder temperatures.
Excessively lean fuel/air metering.
Combination of all.

Pull a few spark plugs immediately after this occurs, and take a reading.
Personally, I'd NOT run or use this until you sort it out.
Detonation (if that's the case) is very destructive.

.
 
Switch to high test gasoline and see if the knock diminishes...if it does, it is probably a detonation issue.
 
A can of Seafoam will remove most carbon buildup in the combustion chamber. Hot water through the carb will remove all carbon build up but you better know what you are doing or you will mess up the engine.
 
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