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random knocking noise

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guatman

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hi all have pair of 454 350 gen 4 with around 500 hours, compression test 150 plus minus 2 on all cylinders, when i run the starboard engine all is good until it get to operating temp of 160 then a random knocking noise happens inside the engine. sortof like random marbles, this boat had sat for over a year and i had the tank flushed and gas replaced. higher revs create same result, random knocking. port engine is fine, both get gas from same tank. had electronic ignition install, knocking was same before and after. weird, any ideas. thanks fyi upon engine start and up to operating temp 160, sounds perfect, only knocks when at operating temp
 
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Welcome Aboard!!

"Random knocking noise" is vague....could be the rotating assembly or ignition related. one thing to verify is the timing advance is correct. Another thing is the oil - when was it last changed, what is its condition and what viscosity are you using? finally, is the noise present only under load?
 
This engine head points we put in electronic ignition The Sounds were the same before and after the oil is Roy tell 1540 just changed it has maybe one or two hours on it the timing was verified when they put in the electronic ignition under load is when you hear the noise or noises when it's at idle there's nothing

UOTE=makomark;604279]Welcome Aboard!!

"Random knocking noise" is vague....could be the rotating assembly or ignition related. one thing to verify is the timing advance is correct. Another thing is the oil - when was it last changed, what is its condition and what viscosity are you using? finally, is the noise present only under load?[/QUOTE]
 
Starting is easy running up to operating temperature at 8oo or 900 RPMs the engines perfect is very smooth once the RPMs go above a thousand 1200 to 1500 or so the rattle becomes more intense but it is still random when I drop down to idle again it goes away


OTE=makomark;604309]does the rattle go away once the engine speed increases, say above 800-900 RPM??[/QUOTE]
 
And there is no loss of power or at least the engine does not seem to lose power I have not put it into gear and run the engine yet with prop moving
 
Sense you have not yet put it into gear as to load the engine, you may rule out “detonation“ as the cause!

What comes to mind for me would be a cracked piston causing a wrist pin slap or knock.

Find and hold the RPM that causes this knock to appear.
One by one remove and replace each spark plug wire from the distributor cap.

This does not always work, but if you remove the combustion process (by preventing the spark from igniting that cylinder) a suspect cylinder may no longer create the audible slap or knock.



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but a cracked piston and ring slap would cause compression problems and all cylinders test at 150 +- 2
isnt that right?? just asking i am no expert, thanks for your help


RicardoMarine said:
Sense you have not yet put it into gear as to load the engine, you may rule out “detonation“ as the cause!

What comes to mind for me would be a cracked piston causing a wrist pin slap or knock.

Find and hold the RPM that causes this knock to appear.
One by one remove and replace each spark plug wire from the distributor cap.

This does not always work, but if you remove the combustion process (by preventing the spark from igniting that cylinder) a suspect cylinder may no longer create the audible slap or knock.



.
Click to expand...
 
Could also be the springs in the damper plate have given up the ghost....there's an inspection port in the upper half of the flywheel cover....
 
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guatman said:
but a cracked piston and ring slap would cause compression problems and all cylinders test at 150 +- 2
isnt that right?? just asking i am no expert, thanks for your help.

It would depend on where the piston material had cracked..... if it did indeed crack.
If below the deck and ring landings, you may still see good cylinder pressure.
Do the spark plug wire test that I mentioned....... it may reveal useful info.
Click to expand...

makomark said:
Could also be the springs in the damper plate have given up the ghost....there's an inspection port in the upper half of the flywheel cover....

This is also worth exploring!
However, centrifugal force will often quiet this down.
But again...... with no load on it..... it could be the source!
Click to expand...
 
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