[i]Originally posted by colorado ...screwing the idle mixture screw in our out doesn't seem to have an effect. On my other older cars, screwing in the idle mixture screw would generally richen the mixture, screwing it out would lean it.
Maybe the mixture screw is not having an effect because the idle speed screw has the throttle plates open far enough that the idle circuit has little effect. Also, turning the screw in will lean the idle mixture and turning it out will richen it.

Also, on another subject, remember the old rule of thumb regarding timing at high elevations. I was always taught to set the timing one degree advanced for each 1000 feet. So here in Cheyenne at 6000 feet, if the manual calls for 6 degrees BTDC, I would set it at 12 degrees BTDC.