have a 79 El Camino with 350, getting a backfire at around 1900 rpms with a load, if I accelerate through it does not occur. Some shutter like a miss. Have checked timing, plugs, plug wires vacuum lines, egr valve gasket replaced. in-manifold bolts checked, carb bolts checked, what else.

Stock motor and carb.? Q-jet?, it sounds like a problem in the carbureter. Smog motors tend to have more complicated carbs that can cause it to do this. how old is the carb and has is set for a while?

no it has edelbrock 650 cfm carb and manifold.

engine was rebuilt 15k miles ago, and carb was rebuilt at same time. ALso curious about components in dist. There is a item that sets under roto rin front part of dist that seems to be cracked casewise. Also want to add only does this when engine is hot