Mower is new, less than 6 hrs running time on it.Stumbles and revs and slows constantly.Uses a lot of gas.Has no adjustments,carb spring is, or looks, correct. Cleaned everything including the air filter and no change.This is the first time ever that I have had a problem with a B&S motor in over 40 yrs.Are they no longer US made?Any advice?Thanks!David