Restring a pgt300 performance power strimmer, it has a white casing which does not come off preventing me from adding more line

Not sure about the fuel/oil ratio. Modern 2-stroke engines are designed to run with less oil due to concerns about pollution and 50:1 isn't unusual. That was the recommendation for my strimmer but I added a little extra oil to maybe 30 or 40:1 and it gave good service for 30 seasons.

The fashion for anti-pollution measures also spread to the fuelling of small engines especially in some areas. In this case the fuel/air adjustments are either hidden behind plastic or metal plugs or deleted from the carb altogether.
If there are no screws it is a preset type and if dismantling and a thorough clean doesn't restore performance it is reasonable to make one of two assumptions (or maybe both) - either the carb is faulty in some way (diaphragm perhaps) or the crankshaft seals are failing and allowing air into the crankcase preventing a good induction.
It isn't unknown for a small 2-strokes to perform well all season and then be next to useless after standing idle all winter. If this is the case it is worth suspecting those seals have lost flexibility...

Go to you tube, on line restring b and s trimmer.

To remove the head to restring, push in the bumper cap/side tabs on the strimmer head, the top should then just pull off exposing the inner spool.

http://vimeo.com/10981278

hi there, this may helps you a bit. Try to unscrew the back casing. visual check the power supply section. If you notice some electrolytic capacitors are bulgy or bloated, it needs to be replaced with same value and type. Felt the power transistors which one really heats up. Over heating of electronic components or part do affects the pictures. this also need to be replace. doing preventive repairs can extend the lifespan of your TV.
Adding exhaust fan may also lessen the heat builds up on the board. Proper ventilation also helps in the performance of the TV. If this do went well maintain the fan.
Please vote and rate. Good day

yes

Hi Bill,
If you turn it upside down you will see the strimmer head. You should be able to remove the protective cover which will allow access to the string cassette. The rest is self evident.

turn trimmer upside down and pull spoolhead off wind new string clockwise

The thing is with strimmers, mower etc.
Depending on which direction the designers have made it spin, they normally have a 'reverse' threaded, head.
Simply 'Tighten' it up to remove, and 'undo' it, to tighten.
Et voila!
Happy strimming:))