[Jack21]

Here's another one that will drive you nuts with Holleys. Fuel leakage from the accelerator pump discharge transition between the metering block, and the throttle body. If the gasket, or casting is faulty, fuel gets pulled out of the acc pump transfer slot into engine through the power valve vacuum plenum. The engine acts like it's jetted too rich because under high vacuum it's pulling in additional fuel from the accellerator pump transfer feed. But, when you move the throttle, the engine bogs because the fuel that should have been in the accellerator pump isn't there because of the transfer feed leak.

Difficult to diagnose. Z7 epoxy can fix with some work with a file to smooth mating surfaces. Drilled slots, brass tubing, and "O" rings can fix this. Drilling metering block and throttle body to use Holley 4175 type accelerator pump fuel transfers will also work.

Bolting on a Demon 650 DP just solved everything. There have been some posts regarding Demon vacuum secondary carbs opening too early or too late, but big engine - light car, such as a Cobra, really need double pumper, mechanical secondary carbs.