I'm guessing this is the stock Nitro Star engine on this? While I'll admit it's been awhile since I ran a HPI engine, when I first got my RS4 MT and broke it in, it seemed to develop a very fine line between being too lean and too rich on the HSN. While I thought I had it dialed in just right, I ended up smoking it pretty quick and it clearly was as a result of running it too lean. As has been mentioned already, get yourself a temp. gun and use it in conjunction with what you see and hear (wish I had back then!) At the time, it sounded good and had the smoke trail out the exhaust, but it had to have been running hotter than it looked because the piston and sleeve were just smoked!
The best advice I got on HSN settings was on this forum and it was simple. Picture your HSN needle as a clock and simply lean or richen it one hour at a time and only after you've gotten your mill up to operating temp. Trying to tune it while its cold will cause headaches! If you lose track of your settings, return everything to factory and start over.
Have fun and good luck!