Ive got the exact same problem. Ive changed my tank (which needed doing anyway) ive cleaned the carbs very well over the last couple of days. changed air filter. And it still runs so badly. Hopefully your post can helps a few of us out with the same problem.

Any rust or contamniation in the tank?

That happened to me with similar symptoms.

Have you checked the spark plugs? maybe they are partially fouled, had a similar problem with mine, ran fine for a while. but the stopped working, only fired on one cylinder on the starter motor.

Reading the thread back again maybe the emulsion tubes are ovaled, or the slow running jets are block or screwed in / out the wrong amount. I think the easiest way to get some joy on this is to find another set of carbs and try those ... if the symptoms change then you know it's the carbs if not the it's either electrical or compression. If it's difficult to cold start then it might well be the valves need doing.

I've had a similar problem - ie poor fuelling at tickover after a few miles running. Particularly annoying with the tall first gear on the SP. Try this - it worked for me:
There should be two short black hoses on a swivelling T piece fixed between each pair of carbs. Make sure they are not tucked up under the air box (i.e. pointing forwards toward the cylinder head) - if so rotate them through 90 degrees so they point away from the carbs (in the direction of the battery) This will ensure you get a clean supply of air which pressurizes the float chambers which in turn will aid low speed fuelling and should solve your problem.