The low speed needles in carbs are usually made by a sub contractor. I imagine they are ground to the taper. Recently, SK engines got some bad low speed needles that was causing the affected engines to run poorly. Last month, I bought a Tower .75 and had trouble with it on the bench going very rich in the mid range. I swapped in a carb from one of my SK 90s and the Tower ran perfectly.

People who have run Tower .75s for years tell me I am just not adjusting the carb right, but since I've been doing this since 1950 and have the SK carbs (and numerous other brands) adjusted correctly, this does not make sense to me.

I might be wrong, but it might be that more than one Chinese engine manufacturer uses the same supplier for carb needles. This could affect several engines of different brands across the board. ST engines are now made in China so some of them may have a bad needle.

If you can't get your engine adjusted where it runs right, return it to the distributor for a fix. These people, from my experience, are interested in keeping their customers happy so you don't change brands.