i'll agree with dodge on the leaning out bit if not nearly running out of fuel completly ...

cant help on the spiting flames and backfiring bit either but i had this same sort of problem with my old maverick ...
everything was spot on - had the dizzy rebuilt, new leads, coil etc ... had the carby rebuilt and she ran like a dream till i'd give it a bit and everynow and then cough cough groan ... pump accelerator ... back fire crackle groan ... pull over ... engine die sit and crank and nothing but fire and run briefly then die ... get out look at everything in the engine bay, wiggle wires and the likes get back in crank and pump accelerator nothing ... 5min later crank it over and off we go like nothing happened ...
had everythin checked and checked had carby removed and checked .... no idea ?? finally worked out waht i needed to do to get it to happen, took the mechanic for a drive in it and then they solved it ... the carb had been rebuilt to suit a standard engine and the thing was just running out of fuel at WOT then heating up and causing the little fuel that was getting in to the carb to evapourate to quickly to be of any use .... leaning out something feirce to the point of stopping ....

but if you've got this happening with 2 diferent carbs i'd start looking at the fuel pump or a blockage somewhere ...