idle problem

[Dark Mold]

Recently bike has become hard to start and idles poorly. Runs OK when at operating RPM
(going down the road). When I adjust idle to good level, it will maintain this RPM for
10-15 seconds then idle down and die. Also, when engine is rev'd and throttle is
retarded back, the engine RPM slows down, slower than normal... It idles back
slowly, then after a few moments, idles down further and dies.
I've removed and cleaned carb (B12), fresh gas, new plug.
Problem remains. I can't seem to get it to idle.
I suspect some sort of air leak or misadjustment in an idle jet/setting. I'm unsure what
to look for. Is there a carb kit available? But, I don't think this is necessary. I'm
more inclined to think something has a slight clog, or there is an air leak...
Looking for the simple fix...