RE: K&B 3.5cc outboard setup on a tunnel help!/?
On a basic carb setup, there is a barrel (roll or slide) that opens allowing air to flow IN with the exhaust is fully open. On the ET, it's the opposite. The venturi (carb) is fully open allowing all the air in while the exhaust throttle closes/opens to restrict/allow the spent gases out. Now with an ET, the normal pipe pressure to the tank does not work. A crank pressure tap is required. Your's has this as there is a nipple on the mounting lugs.
Todays carbs are much much better than say 20+ years ago. Getting a proper needle setting is a bit harder on a ET. But it can be done. Give it a shot.. If not, then find a carb and see if you can tell a difference.