What to do about a ‘slightly’ rough running engine

I always look forward to hearing what you have to say, so here is my question: I fly a 1954 Piper PA-22/20 with an O-320 (no suffix) Lycoming, with about 975 hours since major. I fly it 150 to 175 hours per year, and keep up with all the maintenance an old ship requires. The engine runs beautifully, takes about 1/2 quart of oil between changes, has no leaks and has good compression. However, when I pull back the power for a long descent (2,300 to 2,100 rpm), I get a “”slightly”” rough running engine. If I reduce further, to 1,500 or so, it smooths out. I had the prop statically balanced at a reputable shop and it seemed to help a little, but I was wondering if dynamic balancing would be the next place to look, or is there another direction?

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Can you identify this engine?

QUESTION: A friend has an O-320 in his hangar. I have only the following data, and am wondering how or where I can use these data to determine exactly which engine this is. Data plate says: O-320, eng #2689-27. The case is cast with the number L-17001-27A. I tried using the Lycoming website, hoping for a serial number lookup table, but so far no luck.

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What’s causing popping noises at low RPM?

I have a high-wing homebuilt aircraft into which I have installed an IO-360-A1B 200-hp Lycoming salvaged from a Lake Amphiban, with 660 TT. My problem is that it makes loud popping noises at low RPM, such as when landing or taxiing. I have a cross-over type exhaust, and have had a mechanic check both the exhaust and the intake for leaks, but this is not the problem. Do you have any idea what else might cause this problem?

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