My weed eater starts but won't stay running. what

have to go further in the carb, If you left 'todays' ethanol gas in the weed eater for more then a few months, it probably messed up the Carb. Ethanol gas is very bad for carburators on any machine

back the carb screw out 1/4 turn.

Dirty screen in carb

Rod, Carb. may need adjusted, try loosening gas cap and start to see if cap vent plugged, there is a in tank fuel filter that you could fish out and remove to see if it is plugged. If still not running remove muffler and clean spark arrester screen, if screen plugged the trimmer will do this.
Check the attached links,instruction and guides, Good luck
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Use a hooked piece of wire to snag the filter through the fuel tank, it should pull off the fuel line.

If it is a pull start, leaving the switch on did no damage to the weed eater.
Gary

just bought a new Poulan 25cc 2cycle gas power weed eater . I can start it with the choke on but as soon as put on run it dies.

Either the clutch is worn out or the flexible drive shaft is broken. Either repair is likely to cost almost as much as a new unit.

get a can of starter fluid and a helper. Start the weed eater and then squirt enough starter fluid into the air intake to keep it running while you move it through the settings. This will rid your tool of the crud that is causing your problem. Joe

you have the string in backwards, or the gear is not ingauged.