High idle when or low idle when cold

[EPorsche]

The car is and 81 911SC with stock CIS. Car starts great and idle at about 700 rpm for about 10 seconds, then goes up to about 1200 over the next 30-45 seconds. If you blip the throttle it back fires occasionally through the intake (lean). Once warm the rpm are steady at about 1000 to 1100 rpm and it drives fine. It restarts fine. Timing and dwell are fine. If I adjust the cold idle to 1400, it will idle down to 1000 rpm over a few minutes. Once warm the idle will go back to 1400 rpm. I suspect I have a vacuum leak but can't find it. The decel valve small line is capped.