REV counter gremlin

[johns3c]

Following a cleanout of carbon deposits inside the throttle assembly on my S3C I'd rather expected a sensationally smooth 1000 RPM idle afterwards... What I got was a 1700 RPM idle and a REV counter which just sits below 1500 rpm with the ignition switched off ! By adjusting the screw on the plenum chamber it was possible to reset the idle to 1000 RPM - but how to cure the new REV counter problem ? From previous discussions it seems 'they all do that sir' is one approach. But surely there must be a logical reason to this. Anybody got ideas/experience here ?
Cheers, John