JB,
You can actually hear the maximum RPM setting of the engine. When adjusting the H needle while holding at full throttle, the sound of the engine will change, almost like it has shifted gears (for lack of a better description). The best way to reach this is from a richer setting, then slowly lean the needle until it just falls into this "power band". If you go a little too far, the engine tends to run good for a short period and then die, as it is "running out of gas" due to the lean setting. Have you ever noticed how a trimmer, chain saw, etc. runs real strong and the sound of the engine changes just before it runs out of gas? This is what you are looking for with the H needle setting.
Hope this helps.
