Jeff;

It may be as simple as a sticky linkage...I'd start with a drop of Triflow or synth oil on all pivot and control points of throttle...being a mechanical carb, the throttle plates need to return perfectly to their stop...if they stay hung up in the slightest, that's all it would take to raise your idle...don't use WD40 unless you would like to repeat the procedure next week, and every week after that...useless stuff as a long-term lube...it's mostly solvent...

Cheers