The Choke plate is at the top of the carb, is that the "butterfly" you are referring to, if so it must be open all the way after engine warms up, or engine will be running too rich.
If you checked it with engine cold then it should be partially closed.
The choke plate is adjusted by rotating the large plastic cap(usually black) clockwise or counter-clockwise.
Has the fuel filter been changed?
There should be one in the frame rail under the drivers seat.
Check the Vacuum line from distributor to carb, make sure it is not cracked or leaking.
This hose advances the spark timing to get more power when accelerating.
Carbs are pretty simple but things can go wrong which is why they need to be rebuilt.
The fuel pump push gas to the carb through the fuel filter
The carb has a needle valve and float, like on a toilet tank, as the gas flows into the "float bowl" the float rises up and closes the needle valve.
For this to work the float bowl must be vented at the top, make sure vents are clear.
A carb mixes gas with air using the velocity of the air and negative pressure(vacuum) the engine's pistons and valves create when engine is turning.
The engine is an air pump, it sucks air in by piston going down with intake valve open and then pushes air out when piston goes up with exhaust valve open.
The carb will have "jets" that stick out in each "barrel" of the carb, air flowing past these "jets" suck fuel out of the float bowl, and that fuel mixes with the air.
(The actual jets are located elsewhere but for easier explanation, the "jets" are what gets fuel into the air stream)
As you open the throttle plate more air rushes past the "jets", so more fuel is sucked out.
If the level in the float bowl is too low then not enough fuel is available to be sucked out at higher RPMs.
Carbs also have an "accelerator pump", probably not your issue, this is filled along with float bowl.
When you press down on the gas pedal the accelerator pump pushes a little extra gas into each barrel of the carb, this prevents a lag time between throttle opening and "jets" adding more fuel.
Slow throttle response would be an accelerator pump issue, accelerator pump doesn't do anything when cruising or slowing down.
This page has pictures and labels for your carb: