Hi all - Have a Craftsman 25cc string trimmer that is 10 years old. In the middle of using it a few days ago it started idling much lower than normal and eventually just stalled out. It would only stay running for a little but and would die out even if giving it throttle.

I've replaced the spark plug, cleaned the carburetor, replaced the main fuel line, along with a new filter in the tank. I started it and it was doing the same thing again. The carb kit I bought also included a new carb, so for grins I installed that on and it ran a little better initially but then it started acting up again, even after adjusting the idle screw and low and high adjustments.

The only other thing I can think of is the coil or magneto is going bad. Is it possible for either of these to start acting up like this where it only produces low spark that would cause an issue like this? I'm open to other ideas on what to check, but getting close to throwing in the towel and just buying a new one as I don't want to keep firing the parts cannon at it.

Is this a 2 or 4 stroke engine??
 
Also, check the return line to the tank, that it's not pinched off AT the tank.
 
When you prime the bulb, does the bulb push gas out of the return line?
 
Check the muffler (and screen if equipped) for excess carbon clogging the exhaust.
 
Re spark, if it's starting, that's probably ok.  Coils go bad altogether (usually), or when they get hot
they'll fail...  
 
.

Hi, it's a 2 stroke. Yes when I push the primer I see fuel flowing through the return line back to the tank. I also pulled the return line off, pushed the primer bulb and saw fuel squirting out of the return line.
 
The only other thing I was thinking is maybe the fuel cap vent is bad...? Can I try running it without the cap on as a test to rule this out?
 
There is no spark arrestor on the muffler of this unit.

You can just loosen the cap to check for non-venting.  
 
You may have to check the flywheel to see if it sheared the key.
Your issue sounds like fuel but if everything is correct in the carb....
 
I gather you checked the little screen INSIDE the carb?

Yes I cleaned the screen, in fact I replaced the whole carb with a new one that was part of the kit. Not sure what else to check.

When tuning the carb, are you holding the throttle wide open and then adjusting the air screws till (hopefully) the RPMs rise?     It may be loaded up with fuel but if it's staying running, that's good as you can adjust..   ....