This car starts promptly and idles well both cold and warm. Idle speed warm is about 900RPM.....higher cold until the air slide closes. When I shut off the engine and its warm, if I restart within 15-20 minutes, the idle speed is immediately about 900RPM. If I wait longer, say 30 minutes to 1 hour, so the engine has cooled some but is not cold, the idle speed will be about 650-700RPM and may surge up and down for a couple of minutes before smoothing out at about 900RPM. Advance is set properly, there is a Crane XR 700 laser chopper instead of points and condensor, plugs are new. I did notice that the plugs have a little carbon residue suggesting rich mixture but my understanding is that surging is usually a result of a lean mixture. Any suggestions on what may be causing the low idle condition on starting when the engine is partially warm?