Dave,
Thanks for the emphasis on using antifreeze in the coolant, I will definitely do that. The exit of the manifold was the first thing I checked. I was expecting it to be clogged with rust, but it was clear. Nevertheless, I was surprised at the rustiness of the water that flushed out of the manifold and the block. Very few big pieces, biggest about 1/4" long, but lots of tiny specks. Who knows how much was left in there. Never having flushed the engine in 2000 hours, I guess I shouldn't have been surprised. I didn't occur to me that rust would inhibit heat transfer to the water, but it makes sense. Next step is a vinegar flush.
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