I know you said you have tried edging hard but forgetting the 'hard' part of it have you tried edging more upwind. Sometimes I find that you can edge hard but if you're not pointing your board upwind you not moving the kite to the edge of the wind window and therefore not reducing the power.

If you are generally over-powered have you tried trimming the kite straps for less power?

ABS - Ass Breaking System

Making sure you have a butt plug to prevent the impending water enema. :)

Apart from what's been mentioned already, you'll definitely find it harder to edge on a board as big as what you have, big boards are typically wider as well which adds to your difficulty of edging. 148cm is a great size to learn how to get up and glide for your body weight, but at your level you should already look at changing your board to a more reasonable size, such as between 135-138cm length X 39-41cm width for example. This will make a big difference in your ability to edge the board harder and therefore control your speed much better.

25knots on a 12 is pretty powered. get a smaller kite!

but you can go down wind. your speed up momentarily then start to slow down the more down wind you go.

my most favoured speed killing technique. send it and jump!.

but perhaps not at 10 hours of riding!

SBK - Hi. There are elements in most of the post which are on the money.
Edging - imagine the board flat. Imagine the centre point (right in the middle) is where you rotate/turn/balance around.
If you put your weight more on your back foot it tips back and vis-versa.
Angle the board to look like it would riding heel side. If you put more weight on the back foot you edge harder behind the turn point this turns you into the wind. If you put more weight on your front foot you turn down wind.

You probably know this already, but the trick comes to thinking what U R trying to do when over powered. Try not to bend in the middle (poo stance) you loose your edging pressure. Try to keep your weight slightly behind the rotation point (longer front leg stance / slightly bent back leg stance). Actively think push down on the heels - not just leg pressure. Bend the back leg more to get a lower stance - even think to try and lift the front foot up a little.

First up practise when you are in ok conditions - track an 'S' path thru' the water using the edging (i.e. on / off).
Second - set your self emergency stops and see how U go.

Having said all this - you can still find yourself too light on your feet to think controlling this is easy.
A really good tip was standing up - you go towards the kite - the tension in the lines drops - you use this to get your stance set and then drop into it.
Check out this clip - 3m 20s mark.. really exaggerates this - Mike flattens his board then loads up to get "pop" that is what we are referring to.


I started on a 157 (they made the sale & I was none the wiser) and it was often too large for the wind condtions - you have to exaggerate - realistically - over 25 knots at your level - is pushing the envelope on your kit - make sure it stays fun (also = safe)

cheers
AP.

Thanks for all the advice guys will definitely try and get my hands on a smaller board. I started jumping on my last session. was only trying to do small jumps but accidentally got launched for a few seconds a couple of times. What a buzz!! I find that I get the uncontrollable speed even in the 15 -20 knot range. When I say edge hard I am trying to edge upwind but I find when I am moving fast I can't push the board deep enough into the water to get that real breaking edge I can get when going slower. I can't see myself kiting but wouldn't be suprised if I often have the poo stance. I will try and work on straightening my front leg and pushing down on my back heel.

With the kite low, try leaning back and pushing your hips forwards towards the bar. This will give you heaps of edging power and will be much easier on your legs. Also turn your shoulders around to face the way your going. May mean letting your waist harness slide around a bit (unless you have a seat harness)

Kitch

another though for upwind and speed control is edging hard upwind before you go too fast. once you let the speed build too much you loose edging control. So... in powered situation crank that upwind angle real hard straight away. yourl choke the speed and churn upwind. Note if you do this underpowered your just loose al speed and stink.

Thanks for the info I am on an 09 kite and rarely use the red and black toggles as I don't notice any difference in depower all I notice is the sweet spot moves. I checked my line lengths when I first started kiting but haven't checked them since so could have stretched didn't even think of that. Now all I need is a weekend with wind to put the theory into practice.