That's classic over-carburetion. If the engine isn't able to accelerate when you dump the throttle, the venturi velocity slows down, and you lose the vacuum that keeps the fuel flowing through the carb. You need to go to some sort of progressive linkage that allows one carb to open only at higher RPM, or find smaller-bore carbs that won't lose venturi velocity when both are open at low speed. It doesn't matter how much fuel the pump can supply, if the gas can't make it from the float bowl to the air stream in the venturi. Make sure the float level is as high as possible without flooding, as the same symptoms can result from even a slightly low float setting.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
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