I have the 16 tooth sprocket and I have done the needle shim mod, and I have had no problem with the plug that I know of. The bike is running so well I have not checked the plug since I installed it about 2000 miles ago.
I have adjusted the screw on the bottom of the carb to give the best idle. It is called the Pilot Screw or Fuel Screw. It controls the amount of fuel entering the intake at idle to about 1/4 throttle. Mine seems to run best at 2-1/4 turns out, but each bike is different. Turning it in decreases the amount of fuel, out increases the amount of fuel.
Here is one procedure that works:
1) Warm up the engine to full operating temp.
2) Turn up your idle a few hundred RPM using the throttle stop screw (basically you want a fast idle). This will make it easier to hear small changes in RPM.
Each time you change the screw setting 1/4 or 1/2 turn or so, wait about 5 seconds to let the idle speed normalize. It usually take the carb and engine a moment to react to the change.
3) Turn the fuel screw IN until the idle starts to drop and miss.
4) Then begin turning the fuel screw OUT. The idle should peak and become smooth. Keep going and look for the idle to begin to drop/miss again.
5) The goal is to find the setting that provides the highest and smoothest idle. If it's unclear exactly were that point is then set to the midpoint between step #3 and step #4. For example, if the idle starts to smooth out at 1 1/2 turns out and starts to get rough at 3 turns out then 2 1/4 turns out should be the correct setting.
When the combination of adjusting the pilot screw and reducing the idle speed (with the throttle stop screw) gives you the best and smoothest idle, the adjustment is set.
Typical fuel screw settings on the GZ are in the 2 to 3 range.
Try riding (putting) around in first or second with the idle turned up. Don't use any throttle. After a couple of minutes just push in the kill switch and then pull the plug. Black is rich in the pilot circuit, white is lean and a nice light brown is just right. Do the same for 1/2, 3/4 and full throttle. This will tell you where you stand for the main jet and needle.
Not a GZ carb but this pic shows the Pilot Screw: 5 is the pilot screw and 6 is the pilot jet. 3 is the pilot orifice and the green shows the fuel/air mixture as it heads for the intake.
