Originally Posted by jessepitt
Will the increased spring tension caused by the needle shim cause a more sluggish throttle response?
The slide will be lower for a given combination of throttle position and rpm. This will be detrimental if it reduces the flow vs. what the butterfly could pass if there were no slide at all. It will be beneficial if the slide was already open more than it needs to be for a given intersection of throttle position and rpm.
Remember that the needle is attached to the slide, so raising the slide for a given intersection of throttle position and rpm means raising the needle. If the slide were higher than is needed to simply match the butterfly opening, then the mixture would be richened from the needle being out of the emulsion tube further while the airflow was limited by the butterfly.