Des Hammill states the following as reasonable idle rpms in his book on Weber and Dellorto carbs: (He states that this info is appropriate regardless of carburetaion.)
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Standard cams about 240 degrees duration and 30 degree overlap, 500 rpm; Mid performance cams about 265 - 270 degrees duration 50 degrees overlap, 800 rpm; medium performance cams about 270 - 290 degrees duration 70 degrees overlap, 1000 rpm; Racing cam 290 - 320 degrees duration 70 degrees overlap, 1200 rpm. He also states that modified engines require a considerable amount of spark advance, 16 - 18 degrees @ 1200 rpm.
I run a TR3 with 280 duration 70 overlap cam and high ratio roller rockers which give a mathematical lift of .427"lift @ valves. It has twin Weber DCOEs, milled head, and Mallory dual point ignition. It requires 32 degrees total advance and at least 16 degrees to idle at 900 - 1100 rpm. The car shakes a little at idle and will not run decently with less advance. It seldom runs on when switched off. I have run this set up for a good 20+ years.