A litle background is necessary. Did you replace the carb to try to fix a backfire problem? Do you have a timing light, and if so, what is the total advance now? Have you plugged all the vacuum ports (including the one in the primary metering block)?
If you can twist the shaft and there is some play there's play in the coupling somewhere. If you grab the shaft thru the hand hole and move it up and down, is there any play?