Thank you for the model number. I have attached an image at the end of this reply that illustrates the exploded view of the engine assembly. I suggest going through the fuel system and replacing the fuel lines, but also verifying that the bolts and screws on the image are snug. If there is a leak within the crankcase, vacuum is lost and the engine will not draw air and fuel into it correctly and a lean-out condition can occur. Dependent upon the severity of the vacuum leak, only being able to run the engine with the choke on can potentially be an indication of this too.
Sears Parts Direct
is a direct link to parts for your trimmer.
Please let me know what you find or how this works out for you.