As controlliner said, fix the o ring. If the HSN leaks then everytime you touch it the leak will get better or worse, which will obviously affect the running at idle. O rings are cheap but as a simple test you could just remove the needle and put it back in with a lot of grease, this should give a temporary seal.
On the Ofna engines the midrange needle is very particular about its positioning, maybe this is the same story? For the Ofna engines you have to set the mid needle approximately flush then remove the HSN housing to line up the nearest hole in the mid needle.
Other things to check would be, correct head shims and plug (are they correct for your fuel and conditions).