A plug chop is a method of using the color of the porcelain insulator (center electrode) of the spark plug to determine what the carburetor's state of tune is.
Put a piece of masking tape on the throttle housing and a piece of tape on the hand grip flange. Use a pen and mark a "pointer" on the hand grip tape.
Mark the throttle housing tape at the pointer for no throttle (idle speed).
Now twist the hand grip to full throttle and mark the throttle housing tape at the pointer for WOT position.
Use your best judgement and make 2 more marks on the throttle housing tape for 1/4 and 3/4 positions.
If you have many miles on your current spark plug, install a new spark plug. An NGK BP6HS is a common heat range plug for most chinagirls. A new plug will provide a clean and pristine insulator to get a good color imprint on.
Run the engine at the RPM's that are in question, no faster or slower, for at least 1 mile, 2 preferred.
Take a mental note of how far open you are holding the throttle by using the tape gauge you just made on the throttle grip assembly.
When 2 miles are done you must hold down the kill button and pull in the clutch lever at the same time and let the engine come to a complete stop. No need to stop the bike though. It is very important to not let the engine run at any different speeds or you will contaminate the color imprint on the spark plug. You can casually coast to a stop or pedal to where ever you are going. Just do not let the engine run anymore.
Let the engine cool down to where you can hold the cylinder and cylinder head comfortably in your hands. It should be barely warm. If it is too hot the spark plug threads in the cylinder head can easily be torn out when removing the spark plug.
Remove the spark plug and look at the color of the center electrode. It should be anywhere from a cardboard brown to a chocolate milk color.
If it is lighter in color the carburetor is running too lean: not enough fuel.
If it is darker in color the carburetor is running too rich: too much fuel.
If it is white the carburetor is extremely too lean and this will cause severe damage to the engine in a very short time.
It is always better to tune the carburetor a little on the rich side for better lubrication and a cooler burn temperature.
Adjusting the needle clip position affects the tune from just off idle to 3/4 throttle.
The fuel jet controls the tune from 3/4 throttle to wide open throttle (WOT).
Carburetors are tuned not by engine speed but by throttle position. This is why you made the position gauge in the very beginning. You now know what needs to be adjusted on the carburetor to properly tune it for the engine speed that the particular throttle position was controlling.
A note on spark plugs:
Using an old spark plug for performing a plug chop may result in a bad test.
Yes, an old spark plug will change color however it may have become permanently stained with any excessive residue that may have been present during it's life and the stain will give a false reading. Whenever in doubt, especially if your current spark plug is black, change it for a new one before performing a chop.
About heat ranges: Spark plugs need to operate at a certain heat range that allows them to be self cleaning but not too hot. Hot weather or very high compression will denote the need for a colder plug. Conversely, cold weather or low (factory) compression will denote the need for a hotter plug.
NGK plugs are rated by the amount if insulator contact length. The higher the number the higher the insulator contact length (colder plug). The lower the number the lower the insulator contact length (hotter plug).
Picture below to illustrate.
Spark plug heat will affect the color imprint. Too hot and the plug will burn off most all of the color imprint and possibly damage itself. Too cold and the plug cannot clean itself and will always look darker than the actual color imprint you are looking for as the residues just build up thicker and thicker on it.
Once again, a BP6HS is the most common spark plug to start with.
