strange noise

[JAV]

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.

 

[66healey]

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?

 

[Deleted member 8987]

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.

 

[JAV]

The carbon idea sounds quite likely, especially since I haven't been pushing the 66 lately. Thanks, didn't think of that.

 

[Patrick67BJ8]

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.

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