Problem may be a weak, damaged or missing spring on the engine that is suppose to return the foot pedal to idle. This happened to me when I first bought my rig (used). At the time I was in a hurry, so I connected a couple bungee cords to the fuel pedal. At the shop, (I hate to admit, years later), they found a broken spring and just repaired it and reinstalled. Removed the bungees and more problem.
BTW, those long cables from the dash back to the engine and tranni are very expensive, jacketed and internally lubed. If you can't find anywhere it might need lube, you might try to discern if there is a missing spring.