Engine stalls upon braking

Float adjustment is spot on. Accelerator pump is set right and squirting as it should. I can let it take off on its own, get up to 15 mph or so and hit the brakes with no stalling in forward or reverse. If I hit the gas to take off and then hit the brake it stalls forward or reverse. The two metering screws at the base of the carb are backed out 1-1/2 turns from seated. I don't have a clue as to what's going on with it.