You may have the idle mixture set a bit too lean. If the idle's a bit lean, it can interfere with high-speed fuel flow. Open the low-end needle a LOT, get the engine running at part throttle and then open the throttle to full. Set the high-speed needle so that the engine's running just rich of peak RPM. Now set the low-speed needle as necessary to get a reasonable idle. Anything below 2,500 is OK. If you try to idle too slowly, you can also get transition and midrange problems.