I have a fluidmaster 400. It has recently started to constantly overflow the pipe. Have tried adjustment to screw, but that does not work.

Purchase Fluidmaster seal.
Turn off stop valve under toilet tank.
Remove tank lid and place in safe place so it doesn't get broken.
Reach in under float. Lift float and grasp shank.
Press down on cap and turn 1/8 turn to remove cap.
(You must lift float to raise lever so cap will turn.)
Replace seal.
While holding your hand over top of valve, turn on water briefly to flush any debris out of valve.
Reinstall cap.
Turn stop valve on and put tank top back in place.

See this web site.

How to fix a toilet: replacing the Fluidmaster 400 fill valve seal (HD) - YouTube (http://www.youtube.com/watch?v=XOlK7X82EUc)

Though I would post the video for all to easily see, Harold:

XOlK7X82EUc

Mark