On a new engine it's tuff to set low idle but 20 tanks should be better than 3400 rpm be careful because the low and high interact any adjustments made to low must be followed by a high speed readjust. As you sneak up on idle try this when at lowest you can go pinch the fuel line if it does not change and then 1sec later speeds up it’s to rich go in ¼ turn. Pinch starts to die lean go out ½ turn but at throttle open 1/8 or more not much needle effect my TT25 idles at a crack can just see a small silt in barrel. BTW.. If you got a air leak you will never get a low idle.
The spring is on left side of barrel if you pull the barrel to the lever side you will notice it goes back to the needle side and rides on the screw than holds it all together.
Rich