1981 Gpz 750. problems over medium high rpm

Good morning guys, writing from Italy here searching for a solution overseas
I have a 1981 gpz 750 that gives me problems at medium-high rpm: when I get to around 5k rpm the bike drowns down, like when you run out of gas and there is no way to pass that limit. Actually, mounting standard jets (37.5 pilot + 110) on Mikuni BS 34 (later, with no hole between pilot and main channel and so with no drilled pilot) and no rubber caps), standard air box and standard mufflers. I tried lowering the clip on the needle, but it doesn't change anything. The carburetors are clean and I have no air leaks (tested by spraying the area with carb cleaner spray). Now I'm going to check again the carb floater level, but I'm pretty sure i did a good job there when I reworked those carbs last time. Could be a main jet problem? 

Do not know if this will be of any help or not. But I have seen some info that indicates the needle and needle jet were different on the carbs without the connection/passageway between the pilot and main jets. 
Anyway here it is

read L-R like this:
Model, Cross Pilot & Plug(Yes or No), Main Jet, Pilot Jet, Jet Needle, Needle Jet, Main Air Jet, Pilot Air Jet
GPz750R1, Yes, 110, 37.5, 4BE3, Y9, 250, 300
Z750L3,4, Yes, 110, 37.5, 4BE3, Y9, 250, 300
GPz750A, No, 110, 37.5, 4BC7, Y8, 250, 300
Z1000K1,2, Yes, 122.5, 37.5, 5FL52, Y1, 85, 350
Z1000J1,2, Yes, 127.5, 37.5, 5FLZ50, Y6, 85, 350
Z1000J3, No, 117.5, 37.5, 5CF58, Y2, 85, 350
Z1000R2, No, 127.5, 37.5, 5C48, Y4, 85, 350
Z1100A1,2, Yes, 120, 37.5, 5GX29, Y1, 85, 350
Z1100A3, No, 117.5, 37.5, 5CF58, Y2, 85, 350
Z1100R1, No, 130/5, 37.5, 5B5, Y8, 85, 350

Sounds simple but have you tried running it with the gas cap open (possible blocked breather in the cap)

yes Wookie58, I got your same thought and so I tried that option,but didn't work.

thank you,gd4now, as well for that list: I have another old serie of BS34 coming from a suzuki katana provided with the connection hole between the pilot and the main jet channel...of course I could assemble that carb set with the proper jets and trying those with this GPZ. but, in anycase, I know (atleast here in Italy) these bikes were factory provided with drilled and not drilled BS34, plus keihin, and so I'm upset as I don't understand why it doesn't work properly with these carbs....

I believe your model still uses the "mechanical" advancer for the ignition, have you checked the components move freely to advance the ignition as the revs build ?