Popping and hesitation on throttle after putting air box back on

[50cc]

Hi guys,

After my aftermarket filter broke off it's hose I got tired of experiments and put my old air box back on. I re-jetted to its original jets - the #30 idle and the #85 main (as in my bike's specs).

Since then I get this strange hesitation on takeoff with few popping sounds from the exhaust and deep drop in RPMs - bike hesitates for like 1 to 2 seconds, RPMs go really low, but then suddenly it accelerates fast and runs great overall. Sometimes, when I decelerate quickly, I hear a single strong 'POP!' from the exhaust. I thought it's running lean and put a #90, but that was too much and bike ran like crap and didn't fix the problem. Now I am with the #85 again since #80 is too lean (I have #70 #75 #80 #85 #90 #95 sizes only).

Mixture screw is at 2 1/2 turns out, exhaust is only visually modified, air box is not modified at all, all hoses are great, I have no air leaks at neither side of the carb and intake is 2 months old, carb's diaphragm is new as well, needle clip is in the middle slot, chops are tobacco brownish, perhaps a hint lighter.

Any ideas?

Thanks in advance,
Ivan