So you suspect it's an issue with the fuel? I'm using a 50:1 blend that my neighbor uses in all his equipment, but I'll admit he uses the cheapest stuff possible...
So I found myself on the receiving end of a FREE ECHO PB-403T backpack blower from 2005 that would run but it was very rough and hesitating. I did a full tune up (air filter, spark plug, fuel filter) but issues persisted.
I managed to get it running properly after replacing the carburetor...