Sounds like your floats may be loose. If your carb is brand new, they could be loose. As you drive the screw down and you dont have enough fuel in the bowls to keep it running. You may also have a tiny air leak somewhere around the pump. This would cause the pump to suck air, and not get fuel to the carb. Do you have an electric or mecahnical fuel pump? If electrical you could have a wire going loose when you are driving. So when you key the power, the bowls fill up until you try and drive, you lose the ground for example and the pump stops working and the carb gets no fuel.
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