is 600 cfm carb on a 400 SBC to small?

[justin4260]

so the PO told me the engine was a 350, so i bought a 600 cfm classic holley. well the truck is hesitating really bad. I've tried tuning it for more vacuum but the vacuum doesn't change no matter what i did. i tried adjusting my timing. i didn't have a timing light so i went old school and just loosened it and turned it until the rpms raised and stopped when they started to lower again. then i lowered my idle down. i have to have my idle high in park that way it doesn't die in gear. well i found out i actually have a 400 small block from a g or b body 70-73. so I'm thinking maybe its hesitating so much because it doesn't have enough air? any thoughts?