Surging problem on a brand new Briggs and Stratten 10HP XR

Hello,
 
I have a brand new piece of machinery that I purchased from "overseas".  It came with a Briggs and Stratton 10HP 306cc engine.  I think this engine was not really intended for the US market, even though all of the labels are in English.
 
The engine number is something Briggs doesn't recognize (19N1370348F1).  Fake knockoff? The emissions stickers passed US customs, though.
 
The engine runs okay at wide open throttle, but as I throttle it down, it surges badly.  Closing the choke about half way makes it smooth out.  So, as I understand it, it is running very lean.  Since it runs at wide open throttle, I'm guessing that it is a pilot jet issue?  We are at about 4500ft elevation, so it should be rich, not lean.
 
Can this be adjusted?  I understand that modern engines don't have adjustments any more.  Do I need to drill out the pilot jet?  Briggs doesn't list a pilot jet as an orderable part, so if I mess it up, I have to buy a whole carb to replace it.
 
The engine only turns a hydraulic pump (the whole machine is hydraulicly powered), so I just run it at full throttle all of the time anyway.  It would be nice to be able to throttle it down sometimes, but if I throttle down very much, the engine dies.
 
Any help or ideas would be greatly appreciated.

I agree, it's jetted too lean... Partially choking it proves that or the carb is clogged (which shouldn't be, being it's new).
 
As we can't look up the carb on-line, can you take and post a couple of pictures of it?
 
 
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Thanks everyone for the help.
 
I actually do have an extra main jet (I stupidly thought it needed the high-altitude jet installed - it made it worse, of course).
 
I have a set of very small drill bits on order.  Once they arrive, I will try drilling the main jet a little at a time, and see how it goes.
 
It does seem to be the 592594 carburetor, or at least the jets for the 592594 fit in this carb.  So, does this carb not have an idle jet?  Is it possible that this engine was not meant to be able to idle?  I know that the engines on generators are often set up to never idle.

Your first post, you state you can "throttle down".  That sounds like the RPMs are indeed adjustable.
 
Just an FYI, I have three 10 HP Briggs engines on generators. 
Again, you cannot adjust the RPMs down, it's 3,600 or off however I can hold the actual throttle down (there is no throttle stop screw) and the engine WILL run at low RPMs.  I'll do this on shut down so the engine isn't sucking fuel into it when turning off from 3,600 RPMs.
 
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Thanks again for the help.
 
Yes, it does have a throttle lever that sits right next to the hydraulic levers (red handle in the attached picture).  It operates via a cable down to the engine.
 
 

It shouldn't do that.  You should be able to bring the speed way down if not to a good, steady idle...