I was given a Ryobi PLT2543Y petrol strimmer a couple of years ago, by a
relative who didn't get on with it. He wasn't alone:
http://www.reviewcentre.com/reviews141656.html

It has more or less worked for a while, but would never run without the
choke, and now will not run for more than a few seconds.
I follow the correct starting procedu
1) Switch on ignition.
2) Press priming bulb 8 times.
3) Pull cord with lever in "off" position until motor starts then dies.
4) Pull cord with lever in "choke" position.

At stage 3, the motor dies after a couple of seconds.
I've tried adjusting the red and white mixture screws. I've even tried
cutting off the plastic guards on the screws to allow more adjustment.
The manual mentions that if adjusting these screws does not work, the
strimmer should be taken to a Ryobi agent for a secret carburetor
adjustment. Any guesses as to what that might be?

My planned next steps a
1) Dismantle carburetor to see if there's anything obviously wrong, and to
try to understand what the red and white screws actually do.
2) Throw it away and buy another strimmer (I'm reluctant to pay a Ryobi
agent to try to fix an old strimmer that has such a bad reputation).

Any suggestions?