Does the 39 PWK carb kill gas mileage?
 

As far as performance goes I think the 39 is by far the best. It has better throttle response and accleration bar none. It does leak a lot out of the overflow like the others have stated. And yes after doing all the mods listed in my sig. the fuel mileage went WAY down. Hope this helps.

Does the 39 PWK carb kill gas mileage?
 

the pwk is not so pricey to get and i really think you should at least try keihin and you'll known for sure then,with tail porting
165 mains jets around and 168 to 170 main race ported,the power is so smooth and it is very efficient for trail crusing speed if clutched properly speak of about.

Does the 39 PWK carb kill gas mileage?
 

It is worth the trade off of losing a little mileage in comparison to what you gain in response and performance.
I have been lucky I guess, never had any of my PWK's leak any gas, and I run them on all of my gas bikes when I can.