It is 90 degrees outside and my Scrambler gets kinda doggy on these hot summer days. I checked the plug and it was black. A few weeks ago I checked it at 50 degrees and it was tan. I was real board so I decided to take the front fenders and gas tank off to get to the carb. I moved the needle clip one notch up to lean it out. I rode it around and it seemed to improve just a bit. Kinda cool riding with no front fenders, the suspension and drive shafts look neat. I ended up moving the clip all the way to the top and now it runs great. I thought it was strange for it to be running rich at stock settings with a K&N.
Also, how often should the valves be adjusted? It is a '99 but does not have much time on it.

 

When the temp. changes the carb needs to be adjusted. You did the right thing by moving the needle. You probably bought it when it was 50º outside right? Thats why it is running rich now in 90º weather. I dunno about the valves, I'm not in tune with the 500's.