nklr mechanical question

[Rev. Chuck]

OK K.O.R.G. I have posted this question to the Sport Twin List with minimum of success (Mee thinks they are all racers with no time to wrench their own machines). Bike: Kawasaki EX 500 twin, 1987 13000 miles on the clock, good compression. 1. From COLD it starts and runs great, no need for choke, turns over once or twice to fire. Very strong. 2. Running about 3~4 minutes some *slight* coughing and barely detectable missing then a cylinder drops. Running on one now. 3. 3~5 minutes later full throttle, barely hanging on, she dies. NO chance for a restart for the next 20~30 minutes and the cycle starts again. Carbs cleaned and float level set. All adjustable mixture screws set to factory. Carbs and related fuel system in pretty good condition no blockages, new petcock, no rust in tank, no water either. Spark good (ouch!), yep it's good! No tank vent issues, bike runs the same with gas cap open or closed. Air filter ok, same running on or off. Sounds like fuel starvation, but there is fuel in the float bowls after death. New muffler and header, so no sleeping blockage there. I'm lost. Rev. Chuck (Wishing the EX was another KLR!)