Emulsion tube there is three different types.... primary... bleed... and primary/bleed..... they screw into the main jet and with cards where there is a jet needle... needle in the slide with slips.... the nthe tube is referred to as a needle jet... they effect from... assuming its a mechanical slide carb with jet needle.... then it effects from 1/4 to3/4 throttle.... they really only matter because it doesnt matter how much bigger it is... if its the same size as main jet theres nothing to worry about.. bigger then main jet... wasting your time.... hope that helps with tuning....