Carb'd engines do in fact suck in fuel while engine braking. There are ways to fix this, but no OEM carb that I've seen employs them. The problem with fixing the fuel flow so that DFCO is available is that idle becomes a compromised item. The idea is to introduce a "hat" to the fuel needle when the throttle valve is closed completely, so that some air still gets in, but no fuel enters the stream, as the air doesn't make contact with the fuel needle to draw it out. This interferes with airflow characteristics, as well, and without an on-demand injection system to keep idle, probably wouldn't work too well. The other way that I've seen/thought about to do it, would be to literally cut fuel flow manually by means of a fuel choke in the carb bowl that caps off the fuel jet or the bowl vent. It's not fool proof, but it just might work enough to say so, and won't affect idle or other airflow considerations, only bowl capacity.
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