I'm guessing an air/fuel mixture issue. I'm assuming - perhaps wrongly - that under load in reverse you are turning lower rpms than when in forward. Did you change the carb before you first experienced this issue or afterwards in an attempt to fix it? Wondering if your fuel
mixture might be too lean/rich above lower rpms. Have you adjusted the idle mixture? Try turning it all the way in, then backing out 1 1/2 turns for starters
. Turning the adjustment screw IN makes the mixture richer; turning OUT makes it leaner. Difficult to tell if you're starving for fuel
or air. After it stalls, removing the flame arrestor from the carb and see if there's gas collecting there. If yes, back the idle mixture screw OUT a half turn to lean out the mixture and try again. Let the engine run a few minutes before trying again under load.
Any other recent modifications? Do you have a PCV valve? A PCV valve will require adjusting your idle mixture a little differently than without.