Yes, they can be adjusted, but you'll pull a few hairs out doing it.
The plates at the base (assuming you have the same Morse clunkers as I do) of the throttles have to come off--two phillips screws. Using a flat screwdriver, rotate the nut at the end of the throttle cable. That will shift the handle. Dont worry about which way you turn it--it'll be the wrong way, guaranteed!--but make a note of which one you turned and which WAY you turned it. Then take it for a ride. Better? (I doubt it.) Swell, turn it a bit more until it's okay. Worse? (Probably.) Turn the nut the other way and try again.
Good luck! And be sure to have a drink ready when you finish. You'll need it!
Jeff
"...reality is not nearly as lovely as the world of Liberal Land. No wonder so many people want to go there." - Tom Sowell