No adjustment. They run on manifold vacuum. The vacuum pump section is in series with the manifold port and is just a secondary source for an assist when manifold vacuum is diminished such as when accelerator is wide open or engine is under load. When manifold source is low the pump valving permits the pump to take over and provides just enough to keep the wipers from stalling completely.
I suspect after 75 years the motor has worn parts and the gaskets have dried. Most likely therein lies the problem. There is a procedure to lubricate the motor but if the motor has already worn that has not been entirely successful for most who have tried it.
There are no rebuild parts available. You can have the motor rebuilt at Ficken Wiper Service who bought up and remanufactured some of the parts. Unfortunately he does not sell any rebuild items. If there is no internal damage such as warped castings etc and the motor was common enough with nothing else preventing a rebuild, the motor will be as good as new afterwards.
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