Edelbrock tuning question 1406 Experts?

I had the same problem with a 1407 carb. It seems with that vacuum you need stiffer step up springs, but unfortunately the stiffest supplied in the kit is 8". I´d go one step richer in the primaries and see how it reacts. Or try to put some shims under the step up springs to activate these earlier (best thing i think because you can keep the lean mixture during cruising, but i was not able to try this up til now, winter here....)

I noticed when i put a 1" open spacer on it this flat spot went away - you can try this, too.

Michael