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strange noise

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JAV

JAV

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While visiting our favorite ice creme shop "Mo Fats" yesterday our 66 suddenly made a loud knocking noise, for 4 - 5 seconds and coughed once. Then continued on with no noticeable ill effects. Checked plugs today and compression; no problems found. My question is this what might occur if a piece of a piston broke off? The engine is 15 years and maybe 30k from the rebuild.
 
I would think the noise would continue if you had broken the piston. (I have done this on a jetski before, noise continued until the motor seized) Maybe a stuck valve or tappet that freed up after a few seconds?
 
Yeah, but not likely at idle.
More likely a full chat.

Now, I've seen a lot of float-by noises in my day.
Sometimes, a chunk of carbon in the intake runner or combustion chamber breaks off, hammers for a few seconds, and spews out the tailpipe.

Or, temporarily gets in between valve face and seat, increasing lash cnsiderably for a few revs.

Sometimes a wrist pin suddenly decides to become "stiff", for a fleeting moment.....


But, with the "coughing noise", my bets are on carbon chunk, probably in an intake valve seat, and the "cough" was it clearing.
 
The carbon idea sounds quite likely, especially since I haven't been pushing the 66 lately. Thanks, didn't think of that.
 
JAV said:
While visiting our favorite ice creme shop "Mo Fats" yesterday our 66 suddenly made a loud knocking noise, for 4 - 5 seconds and coughed once. Then continued on with no noticeable ill effects. Checked plugs today and compression; no problems found. My question is this what might occur if a piece of a piston broke off? The engine is 15 years and maybe 30k from the rebuild.
Click to expand...
The noise and length you described is jujst about what happened to me when I lost a piece off #6 piston the first time. Lost a piece of #6 again on another engine and it lasted for about 20 seconds and went away as a soft ticking sound like that of a valve needing adjustment. It last for 14 years until it finally broke off a larger chunk. Lots of carbon will give you a high compression reading in an cylinder.
Patrick
 
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