The brass button is the main fuel nozzle, controlled by the high speed needle...Most Walbro carb have three little holes drilled in the venturi, opening one at a time as the throttle is opened from idle..Popoff pressure has NO effect on fuel coming out the carb....I have noticed the fuel coming out, maybe a velocity stack with the angle with the high part to the rear would help.
I have the latest Walbro parts book, there is no mention of the WT657 anywhere, no price and no mention in the applications section....
The carb might be a special version for a chainsaw, where the engine is operated at high rpm most of the time..
I didn't like the stock carb, don't remember if it was a 657, but used a few different models...There was some blowback, but not excessive......