I sectioned a couple of old 1st Gen Hitachi carbs to see about making a new air/fuel mix needle and noticed that these carbs had significant damage to the needle seat. Here's a little info on the importance of not overtightening the air/fuel mix needle and what happens when someone does. Basically it damages the diameter of the hole in the carburetor and affects how the carb adjusts (or if it's damaged too much, makes adjusting impossible).