Haven't been in the hobby very long. running a Saito 91 with an OS F glow plug. I flew today and noticed that the glow plug has a black deposit not on the coil but the very base of the plug that extends into the cylinder. I didn't notice that last week and wonder if it means that I'm running to lean or maybe too rich. I using 15% nitro and all synthetic and for after-run, Marvel Air tool oil. Also notice that there is dark discoloration on the muffler. No doubt the Saito and muffler run hotter than my os46fx. Also noticed the same dark stuff on the plug but both engines run great. Thanks in advance...

Ed