Back when these were running up & down the road every day there was a service item for this problem. It was a small spring that hooks over the idle speed screw & hooks on the screw on the air horn. These are still available. When you see the card with universal carb springs at the parts store it has some of these in it. It is a small short spring with a loop on one end & a 90 degree bend with a loop on the other end. This will help it return to idle. Make shure there is no binding in the linkage. There is no adjustment on the linkage so sometimes you have to thread the end of the rod & put an adjustable swivel on it.
