The usual cause for this type of problem is a build up of sediment and scale from the fuel and the tank itself that will partially cover the main jet in the carburetor which sits at the lowest point in the float bowl. This diagram will show you where the different circuits in a carb take over from each other. As you can see, the main jet starts flowing fuel just below 1/2 throttle. This is when your fuel will start to lean out if you have the buildup of sediment in the float bowl. To drain the float bowl without disassembling anything, this is how to go about it. If you look at the bottom of the float bowl viewed from the horn side of the bike between the cylinders, there is a small brass screw there. Take a long thin screwdriver and open it up a turn or two. This will purge the float bowl right below the high speed jet at the lowest point in the carb. The fuel will come out a drain tube in front of the rear wheel so you may want to do this outside. Make sure you have the petcock opened when you do this so the rush of fuel from the tank will help clean the bowl out. Run a few ounces out and repeat this again, then try the bike again at speeds to see if it solved the problem. I would open and seat the screw lightly a few times when you are done to make sure it is flushed and no debris is in the seat of the drain screw. Here's a pic of what you are looking for. It will give you an idea seen at the bottom most area of this pic. It's wouldn't be a bad thing to do at each oil change to keep the carb clean.