Poulan String Trimmer

I am working on a Poulan String Trimmer Model PPB100, the guy said it hasn't ran more than 30 minutes, but said he has had it his shed for 3 years. The problem is I can get it to run fairly descent on the bench, but when it is put in the working position, ( motor up, string head on the ground ) it spits and sputters and runs terrible, does anyone have any ideas on this??? Thanks Ed