I put a piece up about this a while back, but I've also got some photos to go with it, so I thought I'd add to it.
First, here's a very good .GIF of how a two stroke works. Watch the reversion in the head cone of the expansion chamber (where the green and grey "gasses" interact) for an idea of how this works and what you are looking at.
This is referred to also as the "pull the pipe" method, because you will get the engine to an operating temperature, make a controlled 15 to 30 second "jetting run", hold the throttle at the setting as you kill the engine with the kill button, and then pull the pipe to check for the wet oil line in either the exhaust port or the expansion chamber. The ideal WOL dimension is approximately 2 1/2" to 3". Less than 2 1/2" and you are lean. More than 3" and you are rich. This has been around a long time in cart racing, and is a well proven method. For example, my exhaust port on my 200 is just 2 1/4", so I'm looking in the pipe for the WOL.
Jetting to the wet oil line dimension
The wet oil line is the boundary between the hot dry exhaust gas and the fresh charge in the exhaust port. Since two stroke engines utilize exhaust port supercharging with expansion chambers the fresh charge is drawn through the cylinder into the pipe and stuffed back into the exhaust port by the return pressure wave. The speed of the pressure wave is affected by the temperature of the exhaust gas. When the mixture is too lean the hot gas enters the combustion chamber and huge pressure rises or detonation occurs. If the mixture is too rich the return pulse is weak and lost power results. Fuel and oil choice as well as timing affect speed of the pressure rise during combustion so those variables should not be changed until an adequate baseline is established.
Step 1) Quarter Throttle
With the engine hot accelerate hard up to 3rd or 4 gear and hold the throttle at the ¼ throttle position till rpms stabilize for several seconds. Pull in the clutch as you hit the kill switch and hold the throttle at ¼ till the engine dies. After stopping pull the pipe and look at the port and head pipe. You should see a dry area in the head pipe and a wet area closer to the piston. The safe dimension is 2-1/2 to 3 inches from the piston skirt. If it is closer than 2-1/2 inches you will want to richen the jetting, more than 3 inches you may want to lean it slightly. Pilot jet, needle straight section diameter and clip position all have an effect in this area. If you are using multi taper needles you may want to test the next increment before making a change. With single taper needles clip position has the greatest effect so you may want make that adjustment first. With ¼ throttle sorted out which may take several runs move to step 2
Step 2) Half Throttle
Get the engine hot again and repeat the test in the same gear as step 1 holding throttle at ½ open. Check the wet oil line dimension and make adjustments to the clip position as necessary to get the wet oil line to 2-1/2 to 3 inches. If your ideal clip position is different from the ¼ throttle you will have to make adjustments to straight section diameter and pilot to get ¼ throttle in line. If you are using multi taper needles it may be necessary to change pilot and or throttle slide cutaway to adjust ¼ throttle readings.
Step 3) ¾ Throttle
Repeat you basic procedures but this time using ¾ throttle. Needle taper is the main variable but main jet will have an effect. You may change taper or get the main optimized and return to this section
Step 4) Full Throttle
This test is about the main jet. With the main jet right you will be surprised at the over-rev hiding in your engine. You will want this test to closely resemble your riding conditions as much as possible. If you are riding in the woods you are likely to be on the main for a few seconds at a time or longer if you are climbing a hill. Desert riders may be on the main for much longer periods so your test should be as close to real riding conditions as possible. Sand riders will use larger mains still.
Knocking, pinging, rattling, or overheating are signs that you are lean, too lean, and damage is occurring rapidly and in a big way. Signs of scuffing on the exhaust side of the piston or a bright stripe from the crown of the piston to below the ring lands or a horizontal smile on the skirt are signs that you are lean somewhere. You may find this occurring as you make changes to your fuel curve going through the steps so you will have to back up and correct your jetting before you go further.
And a little on two stroke plug reading for the Wide Open Throttle settings (main jet only).
It's hard to see down inside this plug (where the important information is), but this will give you an idea..............