Make sure you use a Resistor type Spark Plug. A resistor plug will significantly reduce RFI (radio frequency interference) with electronics. You will notice the "R" in the spark plug number. Electrode gap of .65mm is recommended. A larger gap results in a higher voltage to bridge the gap and also higher voltages result in more radio interference (RFI), also check to make sure that the plug boot is on all the way as this will also cause RFI if not properly seated. The sparkplugs that come standard & work best in the zenoah engines are:
Some guys tune thier engines to sparkplug colour & temperature, I personally do not. I simply use my ears to hear for that clean exhaust/engine note & keep my eyes on the tail. You will notice a smoother tail as you tune towards an ideal running engine, use spark plug colour as a guide if you are unsure what state your engine is at.