It sure sounds like it is becoming too lean at full throttle. Possible causes are split in fuel line, obstruction in tank lines (crimp), or maybe a mechanical problem in carb. Since you say it runs fine at half throttle, I suspect the cam is timed correctly, and the valve lash is close enough that it should throttle up OK. If there was some kind of an air leak in intake attachment, it would have shown up at the midrange throttle setting, so I don't think that is the problem, so again it leads me to believe it's a fuel delivery problem. When you observe the throttle barrel rotate as you open and close the throttle, do you see the barrel moving in and out properly (cam action to open up flow of fuel as it is opened)? If you have another fuel tank and lines, switch it just to rule out that possibility, and make sure it is mounted level with (or maybe 1/4" above) centerline of carb spray bar. Maybe clean out the carb assy to make sure there is no obstruction as well. Not exactly sure what you mean about the low speed mixture buggering up the tickover, might have to elaborate to us "Yanks". Keep us posted, curious what it will turn out to be.