Holley 350 rich as can be.

If you do not get any change with the mixture screws or they are all the way in you may have exposed transfer slots. If this is the case you will have to drill some small bypass holes in the throttle plates. Other issues could be the main body is not square on the metering block surface and you are having some leakage around the gasket surface. This would require some filing of the main body surface.