Well, I know there are several things wrong with the new carbs that can be had these days...loose screws, trash in the float bowls, fuel inlet nipples pulling out just to name a few, but what I have seen the most of is the sticking throttle problem especially on the 30/31.  What happens is the throttle is opened and will return to a degree but not all the way.  Sometimes it will hang up pretty wide open (1/2) and that is no fun for anyone!!  In the following pictures you can see what I have found to be the problem and the fix.  I'm not sure of the cause..poor quality control or damage during shipping, but I do know that it is not limited to a single supplier.  The accelerator pump has a interference fit on itself.  The actuator arm hits the pump housing when the throttle is opened.  Because of the design, when the throttle is opened the pump arm can be forced into the housing but when the throttle is released the return spring is NOT strong enough to overcome the jammed accelerator pump and results in a partially closed situation. You can put a much heavier return spring on but that is not a fix and will result in premature carb failure, it also doesn't always work.

here is the side view of the carb showing the pump and linkage.

  This is the area that needs work.  I just used a 3 inch cut off wheel to clear the gap a little but I would imagine that a small file would work just as well.  Remove just enough material to allow the actuator arm to move smoothly with no resistance.
  When putting the pivot pin back in you need to support the housing as close to the pin as possible and watch the end of the actuator where the pin ends, if it is not lined up right it will be bent and you will be right back in trouble with a sticking throttle.
  That's it. It should be nice and free now and can be installed back on the carb.  I'm sure you could always return the carb and get another, but you may end up with the same deal all over again.  Hope this helps someone!