Not having the specific model number all I can do is provide general information for 2 cycle products. There are several things to look at, the air filter, the fuel filter, the fuel line, and the spark plug.
If the air filter is dirty it can reduce the amount of air getting into the engine. Without enough air under a load the air fuel mixture becomes too rich and the engine dies.
If the fuel filter is dirty it reduces the amount of fuel getting to the carburetor allowing the engine to run with no load but as soon as you put it under a load the engine is starved for fuel and dies.
If the fuel line is brittle or cracked it is probably leaking air, just enough to let the weedwacker run with no load but, as soon as you put it under a load the engine is starved for fuel and dies.
If the spark plug is slightly fouled it will not have a hot enough spark to keep the engine running under a load.
You can purchase any parts you may need from
Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired by a Sears technician. Here is a link that will provide the nearest drop-off locations:
Sears Home Services
I hope this helps.
If I can be of further assistance please feel free to submit any questions you have along with the specifics.
Thanks for using Manage My Life.