The mix screws respond best and it's easier to detect changes if you're running really, really slow, even below the 1200 RPM idle speed sometimes. Too fast and you won't notice much change. It sounds like your idle sync is off, the strong side running way faster than the weak side. I would try to equal the pressures coming out the exhausts by playing with the idle speed screws, then try the mix screw adjustment again. If you can get the exhausts equal then you may want to back each speed screw off the same amount and lower the idle speed to 8 or 900, then try the mix screw adjusting.
Also, incorrect float settings and fuel levels in the bowls can make the mix screws unresponsive.