[Jared at Pelican Parts]

The vacuum created from the engine running causes the sensor plate to rise in it's bore. The sensor plate is connected mechanically to a control plunger which allows fuel to be distributed to each cylinder. So, the more vacuum created (i.e. pushing the pedal down) the more the plate rises, and more fuel is distributed via the control plunger..

there shouldnt be a great deal of resistance in the action of the plate..

The plate itself might be misaligned in relation to the bore. If this happens, it can drag a bit. However it's tricky to get the plate perfectly centered.

The other thing it can be is that the control plunger may be binding. This is usually caused by dirt or gummed fuel getting in there and preventing it from moving freely. I've heard that it is possible to remove the fuel distributor (on the top and back of the airbox with the lines coming out of it) and use compressed air to blow the piston out, then carefully clean the piston and the bore...