Agreed. Iíd check the fuel flow at the carbs. Do you have a tank filter ? Could be clogged or partially, or just inadequate flow. Make sure your tank is venting, that vent line can get plugged too. How short and wide open is the exhaust ? That might be causing the noise when you close the throttle . Think about raising your float level, if the carbs are getting enough fuel in the first place. What mains did you run with the 39s ?
Iíd start there, but now you have lower velocity at partial throttle so you might end up going smaller. W heads with short exhaust and 41ís, Iím not sure if thatís an ideal combo, but Iím sure you can get it to run well.
Fuel filter: I have a brand new fuel filter I will replace right now.
Venting: Is the vent line also also know as a breather pipe? I tried looking for it on a parts fiche diagram. I couldn't find a "vent line" but I did find "breather pipe" / #22 in the parts fiche. Pic attached.
Exhaust: Pic attached. I also pictured the header because the cross-section is substantially wider than OEM and maybe it's important for your to see it.
How much can the exhaust affect fueling at high speeds? I know it can affect cylinder scavenging and valve closure but I figure our desmo heads have return springs for rockers anyway so maybe it doesn't apply much to our bikes. I have V heads with oversized valves, IE cams, polished and ported to match 41s ready to be installed when I have time. Maybe the exhaust will be less of a variable for fueling in that situation. I'm saving up for custom cans anyway, this setup is temporary.
Float levels: Doh! I totally forgot about the float level! I'm going to sync the carbs right now using the drill-bit method. I will adjust the float level while they're off.
Mains: I am 99% sure I ran 155 mains on the 39s which are same as the 41s. I need some time but I can verify that.
The issue seems to only arise at full speed at WOT not partial throttle, which leads me to believe there might be a fuel issue and also that issue is unique to variables involved only with the demands of higher flow rate such as a fuel filter or like you had mentioned float levels. I know the 41s have issues with lean bog at low RPMs presumably due to the lower vacuum not being enough to pull in adequate fuel thus the need for the accel pump, but even so that pump seems to need tuning. Next project