For Clarity: I do not recommend running without an air filter either... just because it's super cheap and and super easy to run one that does not restrict air flow... plus they look sorta cool too... so why not?!?
That said... this gentleman does make some valid points... and I do believe that we're probably overly-anal about these tiny little motors... but that's not entirely a bad thing.
Nope...once she warms up she runs fine at lower RPMs... and even idles fine. Every time though at about 15.5-16 mphs ... it's right to 4-stroking where it stays. I've tried to "run out" the 4-stroking but even after a good 20-25 seconds of full throttle...I was up to maybe 20-22 mph and it was still 4-stroking...no end in sight...not even a BLIP of proper firing.
What is a little odd (to me anyway...but I admit I am a newb at this) is that it's supposed to be 70 jet in there... but my speed carb is running a 72 and it's not a problem.
The BoFengs come stock with the #70 jet...I am assuming your at a 3000 ft altitude thereabouts and that your needle valve C clip is at the second notch to the TOP of the needle??? If so, Move it up to the very top notch and see what happens.
One thing I have different on my BoFeng than you do is a much better, non restrictive airfilter that is a no fuss direct replacement for those stock ones...Less restriction, more air, is much better to run at altitude.