I replaced the spark plugs. Before, I used NGK Iridium IX Spark Plugs. (P/N : BPR6EIX)
And currently, I use NGK Iridium MAX Spark Plugs. (P/N : BPR6EIX-P)
Please see below.
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BPR6EIX-P has a long life and the other performance is the same as the BPR6EIX.
Before
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After