power loss

[GZFORHER]

Hope you guys can help. I bought a 2007 GZ 250 for my wife to learn to ride. Just trying to make things simple for her and the bike was a good deal. It had been sitting mostly unused for at least three years but only has 4300 kms and seemed to run great.
Unfortunately what is happening is that the idle is erratic straying from 700 to 2000 rpm at any point. When it gets warm it has a bad spark-knock when leaving a stop and it keeps having a momentary power loss at higher speeds. More of a quick jerk than a real loss.
I've run a bunch of Seafoam through it and am now running premium fuel with a detergent. The idle seems to be better but the high speed jerk is still there.
I suspect that it is running lean but am unfamiliar with this carb design and don't really know where to go.
Any help would be great. My wife is spooked and refuses to ride it. I'm afraid it's going to melt down.
Long story. Thanks in advance.
I would also like to know how many teeth the drive sprocket is supposed to have. It was supposed to have been changed.
Thanks