String Trimmers

I have a question on a Huaqvarna 128ld trimmer. The problem I am having is that the unit will run fine after it starts and when you go to give it power it will stall and sometimes shut off. I have checked the fuel cap by cleaning it to be sure that it was ok to use. After doing all that it did not help the problem I am having. I have checked all the wiring and didn’t find anything wrong with the wiring. Any help would be great.
vargasp