My opinion would be 2 things.
1.) The secondary air flap is snapping open too fast and allowing more air than fuel and for a few seconds, the air flow has to pick up fuel and then it takes off. The spring tension on th flap may need to be adjusted/tightened up just 1/4 turn and then test. If it improves but still has a little bog, go 1/4 turn more and test. Repeat if needed, but it should not need very much.
2.) The gas is spilling out of the secondary rod holes where the rods go through the top of the carb. These are not completely sealed and when accelerating real hard, gas gets sloshed back and out these holes and spills into the carb creating a rich condition until it is cleared out by being draw down into the manifold and burned off. Float leve could be too high, or even fuel pressure too high.
Simple trick that can be used to seal these holes, if it comes down to it, is to remove the carb top, lay a piece of celophane tape over the gasket/holes, install the top, and puncture holes through using the secondary rods so as to create a snug seal around the rods.
Does not sound like you are running out of fuel or when the engine power dives, it doesn't really pick up - thus "noses over" feeling and power loss. I think your RPM's are too low for valve float. So I would look into a carb issue or adjustment.
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