Flutter below 2k rpm

spitballing here, but if it made more power with a bigger carb it probably means the AVS is a touch small. Counterintuitive as it seems a to small carb can run rich while to big will be lean. Say that because it has higher air velocity through the booster than it should, pulling fuel through the booster sooner, it could spike rich for a second. Small blocks seem to like big carbs.