Ok sorry if i wasnt very clear, at an idol its like the cylinder is barely operating then as im accellerating under load it sounds pretty normal then once i hit my desired speed and im not accellerating anymore its like it goes away again.
I`m no expert, but it sounds like a carb problem. Maybe a dirty jet.
My only suggestion is to try draining the rear carb bowl.
Spray some carb cleaner in the drain to back flush the jets.
Drain the cleaner from the bowl.
Fire it up and let it run until the rear cylinder is fueled up and running smoothly.
Then a test ride.
Hope that helps.