Well, there is only 4 different spots where air can get in to a engine. 5 if you count where it is supposed to come in
Make sure everything is tight. Screws on the back plate, carb pinch bolt and head bolts.
If all those are tight, you may have a pinched of cut o-ring. Back plate, pinch bolt O-rings ( one or both sides) carb o-ring.
Make sure the head is sealed properly ( not cocked off to the side/ it is sitting flush all around).
Fule lines, tank and lid seal, exhaust gaskets ( both) pressure line to the tank
Could possibly be another bad front bearing.....
Or the carb is the issue. Something in it, slide not sealing, main needle and housing ( loose or cracked) as well as the O-rings on the needles...
Do you have any oil/fuel leaking or pooling.....