Our privately operated J160C (2200) has approx 610hrs over 17 years. Over the last 20hrs the engine has been running rich, as when removed the NGK spark plugs are a dense sooty black (prior to that they'd always been light tan). The only work / mods to the carby have been; in 2008 per JSB 018-1, Idle jet #35, Main jet #245 & Needle #290. In 2022 the diaphragm was replaced. Until about 20 hrs ago we used Avgas but since have changed to Shell Premium 98. The fuel level in the bowl is, as per the book, 13mm below the top of the bowl. Advice / suggestions please. Thanks, Bill