The shriek under the hood was a little harder to track down than I expected. I first suspected a vacuum leak in the stack of plates and gaskets under the carb, but an investigation turned up nothing. Then I thought that perhaps the increased height of the carb with the new spacer was binding up the throttle linkage. (The noise did have a kind of metal-to-metal component to it.) There was a bind, but it was minor and not the cause of the shriek..
I finally broke down and removed the heat-isolating spacer and the noise went away with it. (I’m getting really good at pulling and replacing the carb.) I don’t know for sure why that plate was causing the noise, but I have a suspicion. The four barrels of the carburetor match up with four corresponding holes on the manifold. The spacer has a single opening, which must interrupt the airflow from the carb into the manifold somehow, causing the shriek. I still think a heat isolating spacer is necessary, (I’m already noticing more difficult hot starts) but I will have to find one better matched.
The silver lining here was in tracing back the throttle linkage. Not only did I correct that small bind, I also discovered that fully depressing the accelerator was not opening the throttle all the way. I reworked the linkage and found a little extra power (not to mention a more positive response to the choke).
Last item for the weekend was a little more clean up under the hood. I haven’t noticed any leaks since replacing the water pump and restoring the valve covers, so I wrapped the motor in old towels and took some degreaser to the inner fenders and the underside of the hood (which was really a mess.) Nice to open the hood see some sparkle in there.