I bought a MK2 Polo from an auction and it ran terribly. Previous owner assures me it ran beautifully when it went into the auction so something really dodgy went on there.

 It shunts when it goes along and this is less noticeable at speed, and it doesn't idle unless the choke is out.

 I gave it a full service and cleaned out the carb and had the jets blown out, but it still runs as described.

 I don't know what else to do.

id look for air leaks , split rubber base at the carb etc

 

 

Yes, air leaks seem favourite, if pulling out the choke helps.

 

Can you hear any hissing sounds, when you stick your ear around the carb area, which would denote an air leak.

 

Also try pressing down on the air cleaner/carb with your hand, see if the engine note changes, if it does, it’ll be the carb rubber flange as Steveo3002 says.

 

What year/engine is it? 
 

It also helps if you can identify the carb that’s fitted, as depending on which one you have,might depend on what to look for next. A maladjusted carb can also cause all sorts of problems.