I have a 360R and its exactly the same. The reason appears to be the reliance on Manifold Air Pressure, over Throttle Position for the fueling at idle. I have made slight improvements on mine, switched to the latest design plastic intake, replaced the rubber blanking bungs to much firmer silicon items, and got the throttle butterfly fully closed at idle (there is a small bypass that allows the car idle smoothly). But its not perfect, say its was 70% after Post Build Check, its now 90%. The only way to cure it is to get the map changed to use Throttle Position over MAP, which can be done by Track and Road, but this makes the map specific to the TPS fitted, because the 360R uses a TPS that has no adjustment, hence the reason the original tune is used by Caterham.