That would be the "AU" vs "AN" replaceable bushings/atomizers the Dellortos can use
Here is the AU vs AN bushing the atomizer tube screws into
The one on the left utilizes one of 3 potential AU atomizers (they have different inside diameters for fuel flow) which looks like this:
Where the AN bushing utilizes the AN atomizers (which also have 3 sizes) which look like this:
The AU is for 2 strokes and the AN is for 4 strokes. The semicircle on the AU bushing helps mitigate the detrimental effects of air flowing backwards through the carburetor on non-reed piston port 2 stroke engines. Whereas on the AN bushing/atomizer there is no barrier needed and there are also added emulsion holes which allow air to mix into the fuel before going into the carburetor throat, this is to address mid rpm fueling issues that 4 strokes can encounter.
If you look at the bushings the AN atomizer has the threads above the hole where the AU has the holes below the hole. This is for a tiny little (~3mm) air inlet that has air either flow up around the atomizer nozzle which enhances the barrier effect of the bushing (AU) or it has air flow down and into holes in the atomizer tube to mix with the fuel before entering the carburetor throat (AN).
If you get a cheap PHBG clone there is a good chance it'll come with AN parts which will making tuning very difficult if not impossible. When buying PHBG carbs it is a MUST to find a vendor that states what jets/needles/atomizers come installed.
Honestly, unless your a masochist, avoid the PHBGs. Sure they can outperform a vm18 when tuned right (and a high quality 21mm PHBG racing clone can be had for under $30) but getting one tuned right is a huge pain in the ass and requires a lot of time fiddling with it.
You have roughly 4 pilot jets, 10-20 needles, 4 needle clip positions, 20+ main jets, 4 slides, and 3 atomizer tubes to choose from to get your tune on point. At minimum thats roughly 38,000 combos. At the very least to get one roughly in tune you'd want 2-3 pilot jets and 5-10 main jets or some solder and a $5 amazon micro drill bit set to drill out the jets to whatever size you need.
Then to get it fully on you'd want to find the best needle for your setup as they control your mixture from ~20-80% throttle.
