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It seems your engine is running correctly - the gray shows good combustion. To confirm, you should pull the plugs and compare color. The plugs should be more of a tan than gray though (depending on the fuel additives used in OZ). As for the black cylinder, its either a rich injector or oil coming past the rings or valve guides. It doesn't take much oil to discolor plugs or exhaust port. Have you noticed any oil consumption? Take a look at the plug too to confirm its black. Try to change the injector and run it for a couple of days. If the plug blackens again, its oil. If not, it was the injector. Keep your eyes on the AFR's too. I like to run mine a bit rich which blackens the exhaust pipe but the plugs show a dark tan.
Good luck
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