Get the tuning kit for that carb and follow the booklet instructions that came with that carb and you should be able to easily fix your off idle stumble problem. These are some of the easiest carbs I know of to tune, and the booklet makes it a no brainer. Edelbrock did their homework. For your application you shouldn't have to modify anything on the carb to get it to work well. Per many good comments on this thread, make sure the carb is healthy to start with - good pump diaphram, floats set properly, etc, then just make changes to the rods, springs, jets, pump rod, per the booklet ( calibration reference chart )and you should be good to go.