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1968 C' Code Coupe
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Discussion Starter #1
Hi Folks,

Long time lurker and first time poster. Thanks for the great resource. My upgraded C' Code is now back on the street after almost a decade of sitting unused outside in So Cal. I am mechanically inclined, but lack the deep knowledge of many members here. A machine shop did a rebuild about five thousand miles ago (before the car sat). The car has roller rockers, Carter AFB, headers, T5, Weiand intake, Mallory ignition, and a few other pieces.

I've driven the car a few hundred miles since getting it running, but it now has a hollow pop/knock out of the passenger side exhaust. The car ran strong before the sound in question developed. The sound stops when I pull the #1 cylinder wire off the plug. There is some valve chatter, or what I thought valve chatter sounds like, coming from the front of the passenger head. This sound developed slowly, but surely, and had been going away as the engine warms up, but is more consistently present now no matter the engine temperature. I am not driving the car any more until I figure this out. The car has lots of new parts, but none seemed to coincide with this sound. I am happy to provide more information if it would help with diagnosis.

Does anyone have any thoughts? I am by no means a mechanic, so please nothing too advanced. I have a solid mechanic on hand if need be. Thank you all.

Ben
 

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Maybe a sticky lifter or a rocker arm nut that has slowly backed further off.
 

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Hi Folks,

The sound stops when I pull the #1 cylinder wire off the plug.
Classic diagnosis of a rod bearing issue. It could also be a blown out header/manifold gasket. Start with the gasket.

Do you have a mechanical oil pressure gauge, if so, what's the reading?
 

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Stuck valve due to gum or varnish deposits on the stem from sitting for a long time?
Put a quart of ATF in the oil to clean the deposits. It will also clean the lifters.
 

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1968 C' Code Coupe
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Discussion Starter #6
Thanks for the quick replies. I can pull the cover and take pictures if it would help. I will check the rocker arm nut when I do that. Is the valve preload the same a a valve adjustment? While the cover is off, should I squirt some Marvel Mystery on the valve and lifters?

I do not have a mechanical oil pressure gauge, but could grab one if it would help. The headers and gaskets are new. I've had a header leak before and it sounded quite different; more of a "tick" rather than a "pop" or hollow "knock." I hope it's not a rod bearing.

Sorry if these are dumb questions and thanks again.
 

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1968 C' Code Coupe
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Discussion Starter #7
I pulled the passenger side valve cover and both rocker nuts were loose. I adjusted them, though each nut tightness felt very different from zero lash. Intake was much looser than exhaust. Pictures (though I don’t know how meaningful) are attached. I also included a picture of the #1 spark plug.

Upon starting the car, valve chatter was gone and exhaust sounded pretty normal, though I those the passenger side had a tiny bit more “pop” that the driver side. It reved in neutral well. However, when I drove it around the block, I got a dull thumping noise from the passenger side of the engine, mostly on acceleration. It stumbled from stop and was uneven in gear. As speed picked up, it sounded better. The thumping noise did not sound good at all.

Thoughts? Thank you.
 

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