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Problem solved on a FX921. Apparently 93 ethanol free fuel is too slow and cool of a burn for this engine. Switched to 87, just as the Kawasaki manual states, and no more fouled plugs.

I have ordered leaner jets, #131 and #137, and will see if the problem still exist. I would love to adjust the idle mixture screw, but you know they would limit access with a plug.

The mower runs great, just a puff of black smoke on cold and hot starts. I have started raising throttle position to 75% during cold starts to allow a quicker volume of air to enter once the choke is set to 0% after the engine hits

It’s hard to actually run a mower harder than normal when they are governed to 3600. I believe mowing thicker grass would actually demand more fuel based on the increased load. I have tried the ngk BPR4ES which is one step hotter than the stock BPR5ES.

Thank you, I am in the process of trying to find an older style vented cap with the same threads and deleting the evaporative vent line and plugging the vent valve.

Yeah, the mower is brand new. No obstructions in the intake to speak of. I think I have narrowed it down to the tank vents not relieving pressure from the tanks. I can remove the fuel caps on each tank and the pressure exhaust outta the tanks. The tank vent valves have a duck bill check valve...

The overall condition of the plug that I can visually see.

Have you experienced a rich condition on cold starts or even hot starts, like black smoke? I have a 2021 gravely Mach with the fx921 and 10 hours that has the same plug readings. I am also getting black smoke on startups. I have installed new plugs and performed WOT cuts with no time idling...