Gotta say this sounds a bit weird, try adjusting the mixture at high revs(H), turn the H adjustment all the way in then turn it counter clockwise one turn, start it up and run until warm then rev it up on max revs and undo the adjustment until the motor just starts to burble rather than rev freely. This should stop any chance of weak mixture - other thing to do is to check the timing by ensuring that the specified distance between the ignition coil and the magneto immpellor is correct - 0.3mm on my 254! Hopefully if fuel and timing are ok, this fault will go!
Steve
February 2009