I'm having the same problem I had with the 400. On sustained WOT it burbles and dies after 2-4 secs. Different engine, carb, fuel lines, filter. Already swapped coils. NExt is CDI and fuel pump. It acts just like running out of fuel....but the EGT does not rise......idle to mid with bursts of WOT it runs fine......get to a stretch where you can hold ot for a bit and it starts to act up.....seemed to be worse the more I ran.
If it's not changed between engines, then it's got to be a fuel system issue. I have a hard time believing that it's electrical, as it sounds very similar to the issues I had with the Moskito.
I wonder if the fuel tank is venting?
Fuel pump could be puting a vacuum on the tank and then starving the carb.
Could the tank lines be partially plugged?
Check the plug cap just for grins.
I hate jetting gremlins. Going thru the same issue with one of my XR50s. Extremely lean on bottom end, rich on top and the midrange is so screwed up that I may have to cut my own needle. What a total pain in the arse.