Look at the carb base, and you'll probably find that one of those slots lines up with a drilled passageway in line with the idle mixture screw. The proper base gasket will also have a cutout for the same reason. Unless there's a clear path for the mixture, the engine probably won't run at idle. By strangling off the air, you're forcing the high speed system to provide the fuel without enough airflow to make the venturi start working.
Jerry
