Backfiring through carb, what could be causing this??

[Gromit]

Great advice above; I'd start with all those; but one that I don't see mentioned and that might be a cause would be cross-firing plug wires.

Cheap way to check for this - wait until dark and then go out and start your engine put it in park and set the emergency brake; then get out and pop the hood. Watch the engine running and look for flashes of light around the plug wires.

I believe this is a possibility for your symptom, though I admit less so than the comments above - but easy to check for - I see a lot of folks just drape all their plug wires all around the engine bay potentially shorting against the block and sheet metal - the factory even took precautions against this..

One of our list members (whom I think all of us would agree obviously knows what he is doing) - He sets a positive example and I'll attach a photo of his - he was my role model for end-of-story plug wire routing.

Also a please see below photo of a cheap zip-tie method for tying up plug wires to prevent cross-firing.