Please say where you got things when you find them faulty!
I have installed/sold many AMAL Premier sets. Since they started factory testing, I have found nothing wrong with any of them and no one I've sold to has reported any problem to me. The earlier Zinc sets definitely needed to be checked - especially the float adjustment and for swarf.
Many (most) sellers are selling generic carb sets and changing the jets and/or slides to match the model you need. If the carb sub-numbers are 300/301 they aren't even for 850s but can be made to suit 750s. So, you cannot say that they left AMAL bad if you're buying them!
The bottom of this page offers a lot of AMAL carb info for Commandos:
https://gregmarsh.com/MC/Norton/Info/CommandoID.aspx
All sets I sell are as they were provided by AMAL unless I'm asked to go through them. With the Zinc sets, I recommended that. I will still to it if asked. All sets I sell either come with #19 pilot jets, or I provide them.
BTW:
* All 850 sets are the same except for the main jet, sub-number on the side of the carbs, and needle position. They all come from AMAL with #19 pilot jets. The spray tube and needle are special for 850 Norton carb sets.
* 750 carb sets vary between 930 and 932, main jets, sub-number on the side of the carbs, and needle position. They all come with #17 pilot jets from AMAL (I include the #19)