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The Problem
Sorry I never posted the outcome.
It turned out my valve guides were completely worn out. My mechanic pulled a valve spring and found the suspect valve was rattling loose in its guide. This was clearly the cause of the tapping noise and smoky starts.
I had redone the guides myself when I rebuilt the engine. I used phosphor bronze thinking it was a very hard material that would last a long time. I could have sworn I read phosphor bronze was an excellent material for my application, but it turns out I was wrong. Phosphor bronze has great heat transfer properties, but wears quickly and is therefore more suited to racing engines that are rebuilt often. Silicon bronze guides last a very long time and are well suited to street engines.
With the problem diagnosed, I had the car towed home, pulled the heads, and sent them out to get rebuilt. While I had the heads off, I also noticed the rocker arms seemed a bit loose on their shafts so I sent them in along with the heads. The heads were redone with silicon bronze guides, rockers were re-bushed and shafts polished. I also installed new valve adjusting screws for good measure.
The tapping sound is gone and I don't get any smoke on startup. I wasn't thrilled about having to redo the heads after all the work I did rebuilding the engine and transmission, but I think I got off easy all things considered.
Thanks again for all the help diagnosing this,
Chris
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