It may be that the idle mixture is incorrectly set: injected Lycomings should 'kick' only about 10 rpm when you slowly take the mixture control towards idle cut-off with the throttle at idle (625-650 rpm). This kick is when the mixture reaches the ideal stoichiometric ratio as it passes towards ICO. This should be checked with the engine at operating temp - ie post flight.
Being in HK, it may also be the fuel vaporising in the injector lines through heat soaking as they sit above the cylinders. Normally this would result in uneven running below 1500rpm as well.