Tighten up your secondary airvalve spring just a tad until the stumble goes away. It's probably set at about 1/2 turn out of the box - run it up to a 3/4 turn windup and it should cure your bog.

Secondary airvalve adjustment is made using the spring windup adjustment screw and its lock screw located at the secondary airvalve lever. Procedure is as follows:

The secondary spring windup is adjusted with a small, slotted-head screw on the passenger side of the carb, right at the top of the carb on the secondary side. The screw head points right out to the side. 90 degrees from this, on the bottom, there is an allen-head lock screw that keeps the slotted screw from turning. If you have trouble seeing it, place a mirror under the area until you spot it. With a small slotted screwdriver holding the adjustment screw, loosen the allen screw about ¼ turn. This will allow you to turn the slotted adjustment screw. Counting the turns, allow the slotted screw to slowly unwind until all spring tension is gone. You can use your mirror to see the spring disengage contact from the pin lever underneath the air horn. If the spring tension was lost after only ½ turn, the windup was too loose. Bring the spring into contact with the lever. Note when it just barely touches. From this point, wind the spring up between ¾ turn and 7/8 turn. This is a good starting point, and will prevent any bogs or hesitations due to premature secondary opening. From here, you can loosen the windup about 1/8 to 1/4 turn at a time and test-drive the car - when the car falls on its face or feels "flat" going into the secondaries, you've hit the max point. Tighten the spring back up 1/8 to 1/4 turn, and you have the quickest secondary opening rate that your car/engine will handle