I agree, there is simply no good reason to ever adjust a gasoline engine while it is running. The best thing to do is get it running at factory settings and listen closely to what it is doing. While the engine is not running but warmed up, adjust so that you get a smooth transition to full throttle. Once you have a good reliable idle and transition to full throttle start flying the engine and take note what it is doing in the air. Most likely you will see that you are still a bit rich and the transition is a bit sluggish. If you get too lean on the low needle it will not want to transition. I adjust the low until I get the hesitation and open it up just enough to eliminate the hesitation or where it requires a short warm up to transition. I will usually sneak up on the high end over the course of several flights. If your engine is new you will notice the mixture richening up as things seat. After about 3 gallons you can get to the point where you will not have to make any more adjustments. Here is where guys go somewhat a stray. Once the needles are set and you have a nice low idle, good transition and good high end leave the needles alone! If you get out to the field and your well behaved engine is not cooperating, you have something wrong that needs to be fixed. It is not repeat NOT the needles. They do not move by themselves. Either you have some debris in the carb or an ignition related issue.