3 things to check
I replaced both control cables last fall on my 81' H30. The 3 things to check are the levers at each end, the cable itself, and the cable brake. I would first trace the throttle cable and see if there is a throttle brake on it. Remove the throttle brake and see if the problem goes away. If it does, then reinstall it and adjust the tension so the throttle moves easily but dosen't vibrate back to idle. If the brake isnt the problem, then disconnect the throttle cable at each end, and see if the engine lever, or pedistal lever is the problem. If they both move freely, then that only leaves the cable. Check that with the cable brake removed, and see if it moves freely (it should). If the cable is stiff, then it will need to be replaced. I got all the parts I needed from Edson directly. Hope this helps.