Why will my weedwacker only run on full out with the choke open?

Help requested for the mechanically declined.

My Poulan weedwacker has a start/run lever. It will start and run but runs full out unless I pull the trigger. If I pull the trigger the lever automatically switches to the run setting and, as long as I keep the trigger fully depressed, remains running. However if I release pressure on the trigger even part way it will begin to bog down and die. If I release the trigger completely and try to pull it in it will bog and die.

Why?

Bad gas? Bad premix? Wrong gas to oil ratio (it is supposed to be 50:1, what happens if the ratio is off)? Bad or fouled spark plug?