Engine misses and pops

We bought a Polaris 90 Sportster for our grandson last month - we have restricted the throttle by adjusting the throttle screw.  In hot weather it keeps stalling - in cooler weather it pops and misses.  We have had it back to the dealer twice - they advise they have contacted Polaris and there is nothing they can do to stop the miss - actually they claim it is a "lean mixture problem" and because the carburetor is preset at the factory and there is no adjustment on it there is no fix for it.  We even dialled the throttle back up and it still does it (after we were told it was only a "lean mixture problem" at low throttle)  This is a gently used 2009 - the dealer advises they have also tried out their brand new 2009 in stock, and advise it does the same thing.  So they are saying "oh well" that's the way it is...that all the 90 Sportsters have a miss.   Needless to say we are really frustrated and disappointed - has been in the shop more than at home - and they say there is no fix.  I can't believe Polaris can build these things with a "miss" and say - well that's the way they are...  Anybody else have this problem? Or even would appreciate a note from anyone with a Polaris 90 that has NOT got a miss or "popping" to the engine.  Thanks