If an engine is too rich it can also mislead you into thinking its lean. Dont be afraid to play with it alot. I also just ran into a problem with a thunder tiger .91 4-stroke that had my head spinning in circles thinking it was rich,then lean,then rich,then lean........turns out I think it was overheating like the other guys said. I've never had an engine overheat before including this one. And I was trying to tune it on the ground and it wasnt even inside a cowl. These hot days can really cause problems. So,while tuning it....dont run it for long periods of time without letting it cool. But be careful because if it overheats in the air you in for a BIG eye opener! I made the mistake of taking off with that thunder tiger I spoke of once I "thought" I had it tuned close enough. It must have decided to overheat or whatever it was doing just as the wheels left the ground. It still amazing me to this day how I managed to get it around for a landing in one piece........

...needless to say,I dont know if that engine will ever see air again because I dont know if I'll ever trust it again. I ended up replacing it with a O.S. .91FX the next day.