etsjw wrote:Been thinking about your problem and I think Eldert identified what is happening - the float bowl is running out of fuel resulting in a lean condition at
WOT in 4th/5th gears.
You could check by installing a transparent fuel line, put the bike on some type of stand so the rear tire is off the ground, run through the gears and observe the fuel level in the line.
As you have checked tank venting and petcock size, then something most be upsetting the action of the float and inhibiting the fuel flow. This might be related to some vibration issue. Can you insulate the carb from the head to try and lower/change frequency of the vibrations reaching the carb?
These kind of situations can be real tests of character!
Good luck and please let us all know what finally worked.
I'm of the same mind as you on this. The motor is sitting on the bench right now about to go back into the bike. I've adjusted the floats and also have a 3.5mm float needle to put in there. If it's a fuel bowl starvation issue then I have to think that it will soon be corrected. If it doesn't then I will be sufficiently convinced the bike is possessed and I will burn it at the stake. Either way I will be sure and report back here.