Sounds like maybe the idle cicuits in the carb are clogged, or maybe just one side. I had the same problem a few weeks ago after I did a head swap on my car. Ran fine before the swap, but ran like crap the first time I fired it after I did the work. was very hard to start and wouldn't run below 1000 rpm. I guess moving the carb around disturbed the stuff that had built up inside over time. I pulled the top off the carb and it was full of sediment. Cleaned it out and it runs perfect.