Weed eater will not start. when it does it will not cut grass. Just knocks grass down. Shaft does not turn fast enough. Idles instead of reving up. Can reach down and grab string and it stops.

check for debris around the cutter

the carburetor needs adjusted.

If the line is spinning but not cutting then most likely you have wound in the reverse direction.

Start with carb adjustment. Turn out the low and high adjusting needles 1 full turn, then adjust the choke.

Check the motor hasn't moved up the shaft and separated the inner drive cable from the spindle.

Hi goldengear...

Make sure you have installed the blade on the right way, if it is installed upside down then it will "beat" the grass instead of cutting it and make a terrible cut, also make sure the blade is good and sharp.
The rough running may be due to trash in your carburetor, I would advise to clean your carburetor and also check you fuel tank for trash, water,etc. if there is any trash,water,etc in there then you need to also clean the fuel tank.
Do one more thing and check/clean/replace your fuel filter if you have one.
Please take time to rate me

This sounds like a problem with the drive shaft (s) or the gear head ...partial disassembly of unit to verirfy would be required.

Take the shaft off at the motor,see if any thing is worn there.Also,take head off of end of weed eater,check for grass or any build up in there.

you need to replace fuel filter inside the fuel tank . fish it out with and empty tank using a coat hanger and a small hood bent on the end.filters are around $9.00