Before you chase all the solutions offered, why not fix the only obvious flaw that you first saw- the leaking airbox breather?
I've had that experience on previous carbureted bikes where the o-ring split causing lean running, backfires etc. It's not always a visual, "that looks fine, the bolts are tight" situation. I also had a crossover hose between carburetors develop a hidden crack where it ran beneath the gas tank. My bike had amazingly developed more power... right up until an exhaust valve seized.Appearance isn't everything. Those particular isolators have a metal base, I can tell because they bolt on instead of clamping on, and as such they use an o-ring gasket between the isolator and head. These get squished over time and shrink or even crack and break. I'm not betting the farm it's your problem but worth a look for sure and it's an example of where vacuum leaks can occur besides hoses.