timing issue or not?

Couple of things.
Flat spots/stumbles,etc mean lean 99% of the time.
It is actualy very difficult to tune so rich as to cause a flat spot.
In a carter/edelbrock the primary jets should generally always be larger than the secondary jets.The metering rods(used for fine tuning) through the primary jets reduce the area of flow - hence why the larger jet than in the secondaries.
As vacuum decreases(such as WOT) the metering rods(which are stepped in diameter)rise to a smaller step thus increasing the area through the jet alloeing more fuel to flow.
If you insist your jetting is correct there is two ways to improve the stumble.
1. Use a smaller metering rod.Your originals should be 7047s? Try,say,some 6842s or 6537s.7037s could work well too.
2.Change to a lighter stepup spring.Yellow works best in most cases.This will get the metering rod to rise quicker thus riching the mixture quicker.
I assume it's a performer?If so you can't adjust the secondary air door without a lot of trouble.
If it's a thunder you can adjust the secondary air door.

For timing I like to see around 12-16 btdc at idle but check that you don't get too much total.

Hope this helps.