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Honda Odyssey unstable idle speed and solution

A 09 Honda Odyssey with 96,000 km mileage was taken to my shop because of unstable engine idle speed and black smoke during acceleration.
I had noticed that when the engine speed accelerated to 1000r/min, the car would exhaust black smoke. So I first checked the engine fuel system, but no leak or too low oil pressure was detected.
Then I tried to pull DTCs from the engine control module with Honda HDS HIM, and again, did not find any suspicious.
I removed the spark plug and tested the cylinder pressure. The pressure was 1,250 kPa, which was within the normal range.
I checked the air supply system. There was no leak.
And finally, I removed the cylinder head, inspected inside, and found two inlet valves in the third cylinder was deformed and contributed to leak. This could be the problem that caused unstable idle speed.
So I replaced the two inlet valves and repaired the valve sets. Then I started the engine and accelerated, but the problems never came back.