Why did the engine idle consistently after the throttle plate stop screw and throttle plate itself was moved a bit farther open? Was it additional air? No. When the throttle plate was moved, the injection pump, via the linkage, was also moved to deliver more fuel, i.e., a richer mixture. The engine was stalling because the mixture was a bit lean. After the adjustments the same amount of air was entering the engine as before, but more fuel!