Could it be an over-rich condition when the accellerator pump duration overlaps the main circuit activation, which may happen to be at 3,600?

I've had issues with a bog/flat spot footbraking and been working on shortening the pump duration. Don't have an A/F meter yet.

Bringing the rpm up to 2,200-2,500 may start the main circuit, but it also reduces the volume and duration of pump shot delivery over what you get from idle.