Zenith 49 tin not returning to idle
 

Hi guys,
I have a problem with my 2.7 running zenith 40 tin carbs, when I Rev the engine the idle does not return to normal until after 30 seconds or so.
I've stripped and cleaned, set up with Zenith tech setup pages and still no change, timing setup and is correct.
I'm stumped as to what it can be, does anyone have any suggestions?