Hmmm...
I could get the idle down if I slowed down and put it in a 3rd or 4th and let the clutch out. Then the idle would stay down. Then when I rode away...
I am by no means an expert T, It sounds to me you may be drawing air through the ignition side crank seal. When you accel the vac is increased causing a lean condition, when you bog the engine, reducing the draw on the seal its able to hold at Idle.
I guess by comparison it's like having bad valve guides/seals and upon decel you draw oil in the cyl. and get smoke in your SB/BB Ford

If the tranny side seal was bad you would be drawing trans fluid in the cyl and fowling plugs/going way rich, and losing tranny fluid.
I know probably the last thing you want to hear is to pull the engine and re-fresh it, however I think by the time you go through everything you will have done so anyway.
You verified your compression is good, and the Prev. Own. told you the top end was done.
If the rest of the maintenance on the bike has left you to evidence of neglect, it would be a pretty sure bet those are the Original crank seals and reeds etc.
When you go through the carb to check the jetting, have a look at the slide and check for scoring and places the chrome may have peeled or worn off, if the bad air filter had been run too long there may also be excess air "leaking past" the sides of the slide.
You may also consider pulling the intake/reed block and have a look. "While you're at it" (You can save a few bucks and make a new gasket)
These old girls are quite reliable when the maint. is kept up, and alot less than a high output 4 stroke, but most guys run the poor things into the ground and then get ticked off when the top end needs service. Unfortunately you have gotten stuck due to the lack of maintenance by the Prev. owner.
I'm not saying he's a bad dude...he just didn't like to work on his bike

I apologize for the brief novel. Once you go through the bike and know it's done right she'll "Love You Long Time"
Tuck\o/