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420 motor knock

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Tim Peterson

09-08-2002 17:51:17




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Looking for some help. My 420 suddenly developed a motor kock. It knocks at all speeds and motor vibrates.Oil pressure and compression normal.




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Clooney

09-08-2002 18:04:24




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 Re: 420 motor knock in reply to Tim Peterson , 09-08-2002 17:51:17  
Tim, we'll need a little more information to help you. Diagnosing a knock over the internet is iffy at best.
~Is the knock once per revolution?
~What does the knock sound like?
~Does it change under load?
~Does it change with the clutch engaged or dis-engaged?
~Does the engine smoke?
~Do you have the same compression on both cylinders?
~Did it just start knocking or did the knock come in gradually?
~Are the spark plugs normal or is one burnt up or oil covered?
~Anything else you can think of?

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Clooney

09-08-2002 18:02:24




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 Re: 420 motor knock in reply to Tim Peterson , 09-08-2002 17:51:17  
Tim, we'll need a little more information to help you. Diagnosing a knock over the internet is iffy at best.
~Is the knock once per revolution?
~What does the knock sound like?
~Does it change under load?
~Does it change with the clutch engaged or dis-engaged?
~Does the engine smoke?
~Do you have the same compression on both cylinders?
~Did it just start knocking or did the knock come in gradually?

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Tim

09-09-2002 11:39:25




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 Re: Re: 420 motor knock in reply to Clooney, 09-08-2002 18:02:24  
I will try and give some more info.The sound is more of knock then a rap. It appears to be coming from the motor and telegraphing to the trany making noise there i have never heard.The knocking started suddenly.It doesnt change with load or if the clutch is engaged or disengaged.The motor has always smoked a bit when holding back.I have the same compression in each cylinder(120). It starts easily and runs good but lacks a little in power from normal. Oil pressure is normal.The motor also vibrates more than normal.I know it could be rod bearings but oil pressure seems normal. Could it be a loose or misalgined flywheel?I do use a brushhog.Hope I gave you some more clues. Tim

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Clooney

09-09-2002 18:41:22




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 Re: Re: Re: 420 motor knock in reply to Tim, 09-09-2002 11:39:25  
Tim, seeing as the noise doesn't change with load it doesn't sound like bearing or piston related [keep in mind I can't hear the noise from here though].
~The flywheel could cause the problem but that should also change with changing load.
~If the clutch in & out doesn't change it, it probably isn't transmission related.
~Try removing the fan belt [a real pain] to see if the noise is coming from the fan bearing. Make sure the front pulley isn't coming loose, use a stethoscope or hose or long rod & listen to different areas to try to isolate the noise area. Make sure the flywheel under cover or oil pan isn't bent & contacting the crank or flywheel. Listen to the governor to see if that is the noise producer.

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Wayne in Tn.

09-09-2002 18:01:27




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 Re: Re: Re: 420 motor knock in reply to Tim, 09-09-2002 11:39:25  
Tim, you could have flywheel problems or rod bearings, but those engines are noted for some noise, probably the wrist pins. If it is not too loud, you may be ok, but if it is excessively loud, I would not run it and go in and see. Usually a wrist pin or piston slap will not be too loud.



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