xj600 '91 dies on open throttle

[drago]

Hello!
in past two weeks i get new problem with my old xj600 1991 - i have it two times so far, on about last 400 km riding.
first time i got it after about 60 kilometers ride, at event time is about a 5 litres must be in fuel tank, i ride slowly ( about 20 - 25 kmh), about 2000 - 2500 rpm, on a unpavned country road. after that i take a small stop for minute, without shut down engine, and see, a idle rpm is about 900 or so ( on warm engine must be about 1300 ), and engine sound is bit strange, with some clunk? after that i ride further, but after about a 400 metres there willl be an attacking dog. i stopped before him, and do some engine revving to about 5000 rpm to scare it, and after a second or so revving i see, engine tend to die - idle drops to about 500 rpm, and after some second engine dies. i start it again, and it dies again then i start it again, and it starts for only about a second or less and die. i try to start it again and again, also with open throttle, but no any luck. i also try to switch fuel cock to "res" and to "prime", but it also not help.
then i put it off, and after about 10 - 15 minutes try to switch fuel cock again to "res", "prime", and see in "prime" position no start fuel flow to carbs. also checked boots for crack, fuses, and found all looks ok. then i wait another 15 - 20 minutes, again rotate fueal cock, and now see asome amount of fuel flowed in carbs when i "prime". then i try to start engine, and it starts, revvs when open throttle, iddle about at 1000 rpm...
i put fueal cock to "open position", and drive in that without any noticeable problems, on highway, about 80 kilometres to fuel station and fill tank up. after that i drive almost nonstop about 140 kilometres on highway, and all its ok. temperature in that day is quite high, around +27 celsium.
on another day i drive to home for about 180 kilometres with short stops, highway, and all be ok. after week or so, i drive to my rural site - again about 70 kilometres, there will be about 8 litres of fuel in the tank - all is ok. but on another day i drive back to city, and after a ~60 km nonstop drive, in city, at the junction, i have that same symptoms.

on odometer from last fueal-up is be 330 kilometres, indicator says there must be about 5 litres in fueal tank. at traffic light i found, idle rpm again start dropping to about 800 or so rpm. i try to some revving engine, and the same time switch fueal cock to "prime", "res" in case i run out fueal, but it no helps - soon engine dies. i try it start, again - it starts for moment and again dies. try to turn out chocke - it starts good and revs go up about 3000 rpm, but when i try to open a trottle, engine dies very fast! try the same second time - the same - as i start open throttle, it die momentary. on third time it not start even on full chocke. i put it on pedestrian wal, and again try to switch petcock to prime and see no fuel flow. fuel stay in fuel line ( mine is clear, from silicone, from petcock to fueal filter, fueal filter case also is from clear plastic, as so i can see fuel flow). i inspect fueal lines for bent, and not found any problem. shake bike fuel tank, lean it, try to start- nothing.
then i got s screwdriver, and open and some drain from carburettors ?float chambers? via drain screws.
then i again put cock to "prime" and see fuel flow. try to start engine - it starts and iddle ok. switch to "main" position, and drive to garage, about 10 kilometres, without problems.
where may be problem? i thinking about float valve on carbs, but as i understand, there is 4 carbs, and 4 separate chamber valve - one for each carb, and all at one time that is very unliked to fault?
any ideas?