Well, if you have manually throttled the engine, you should be able to feel the return spring on the throttle plate. I assume it's good since you didn't mention anything wrong with that. Next step is to take the intake boot off the throttle body and inspect the body and clean it with carb cleaner or something similar. See if that helps.
If not, you may have issues with the actual throttle cable acting on the throttle and the best bet is to follow it and see if you can find a kink or see if it's corroded.
Are you getting any codes?