This carb has the primer bulb and the diapharm vent is in the plastic housing, partially covered by the bulb retainer plate.
Engine, Echo 21cc, ran fine on stand but I ended up making a longer throttle arm and installing it upside down (needle down). Engine now runs like cr@#. (starts fine but will not power up). Put my hand behind carb while running and got big RPM improvement. Figured a velocity stack, homemade, would help and it made it worse. No throttle response.
Stack mod A was pointing aft; mod B got 45 degree cut to where throttle plate was visible. Mod B got the throttle response back but still low RPM.

After thread searching, I think the problem is prop wash directly on the diaphram vent hole. (Open to other ideas)

I do not have any metal diaphram cover plates to modify. (On-hand Zama are all wrong) Any suggestions on method of attaching a vent line into this plastic?

Pics to follow...dinner

Most comments on using velocity stacks advise to place major opening side aft.
What is the reasoning? If the diaphram vent is aft I understand, both would be somewhat blocked from prop wash (non-cowled opening). If 'vent' is forward what problem(s) should I expect if I rotate the velocity stack to face forward where both intake and vent encounter same prop wash?

I rotated velocity stack (now faces forward) and got somewhat improved RPM, have not finished carb adjust yet.