Honda GC160 on Craftsman Tiller -- CARB PROBLEM

I have a Honda GC160 engine on a Craftsman Tiller.  It has worked perfectly for 10+ years, and I always do the routine maintenance.  
At the end of last season, it was running a little rough -- with lower than normal power.  I figured I'd deal with it THIS year when I do the regular maintenance.  
So I started -- and completed -- the regular maintenance.  I can get it running, but it has the same problem it had at the end of last year -- no surprise there.
 
It appears to be a CARB PROBLEM.  Here is a diagram (from the shop manual) of where I think the problem lies:

With the engine off, the top of the governor arm is pulled all the way toward the right (toward the carburetor).  This is the case no matter the position of the choke or speed lever.
 
On starting the engine, the top of the governor arm moves (somehow) all the way toward the left (away from the carburetor).  This is the case no matter the position of the choke or speed lever -- except, of course, with the speed lever in the OFF position.
 
If I lift the speed lever to the FAST position, it is pulled all the way toward the right (toward the carburetor) -- but only for a half a second.  The engine revs normally, as it should, but then moves back toward the left and revs down.
 
I can manually push the governor arm toward the right (toward the carburetor), and it revs up as it should -- but it won't stay there, even if the speed lever is set to HIGH.
 
I can move the governor arm easily with my finger, and I've "WD-40"-ed everything.  There is no dirt, grime, goo, or obstruction that I notice anywhere on the governor arm.  The lower governor spring looks in perfect shape, condition & position.
 
Like I said, I've done all the routine maintenance, and I can't figure out what to do.  I tried adjusting the carburetor throttle stop (AKA idle) screw (NOT the pilot screw), but it was in the correct position already.
 
Any ideas at all would be appreciated -- even if they don't solve the problem.  I've tried everything I can imagine.
 
Thanks!!!