Please visit the original source of this information here: http://tech.flygsw.org/walbro_tuneup.htm for even more detail. The most relevant parts have been included here in case the original gets relocated.
Some basic terms (see diagrams). The lowend needle refers to the one that trims idle mixture mostly and affects mid-range mixture somewhat. The highend needle trims mixture at mid-range through full power. Unscrewing the needles on this carb increases fuel flow which richens the fuel/air mixture. Screwing them in reduces fuel flow which leans the fuel/air mixture. Don’t confuse fuel/air mixture with fuel/oil mixture which refers to how much oil gets poured into each gallon of gas.
First, screw in both lowend & highend needles all the way then unscrew to about 1 3/4 to 2 turns out each. Choke or prime the motor until the carb is wet. Start the engine and let it warm up. Let’s set the topend first since it’s the easier of the two. Go to full throttle. Adjust the topend needle for peak RPM. Leave it wide open for about a minute to see if it changes any.
Should the engine go lean, open the lowend needle slightly, if this dosen’t work you will have to adjust the needle valve inside the carb.( I will explain this later) If the topend runs OK, then slowly pull the throttle down until the engine begins to “four cycle” hold the throttle there. Adjust the lowend needle until the “four cycling” barely stops. Now lower the throttle more until it “four cycles” again, and adjust the lowend again. Keep doing this until you reach full idle. Now, from full idle begin to throttle up until the engine starts to bog or hesitate. Open up the lowend needle just enough to eliminate the bog or hesitation.
When this is done right, you will be able to set the throttle in any position and it won’t four cycle, plus you will be able to transition from idle to full power without any hesitation at all. Now, for easy starting it’s best to have the lowend a little rich and it will four-stroke a little.