popping and backfire over 1/3 acceleration

Hi,
I have a VL 1500 LC from '99. I've bought it last year in spring, run it out all year. There were some slight problems with the carburettors, sometimes no power and severe backfiring, other times no acceleration regardless how muck i turned the handle. So in this winter I took it apart. Dismantle completely the carbs, cleaned them, reset them to factory set up (3 turns out on pilot screw and 7 mm height an float.) then put them back, synchronise them. Changed the sparks with some k&n and air filter with performance K&N, the washable one. Had some small problems to decompression but after cleaning the coil and lube the cables and adjust the tension all good. It starts perfectly even in cold weather.
All done...let's test. Well the test was running perfect. Good response from acceleration, good tork and power. But when i accelerate above 1/3 disaster. It pops like a machine gun and the engine chocked. I almost hit the handlebar with my chest. I've tried to accelerate very slow, same. Until 1/3 of accel it runs perfect. After that it's a mess.
Pipes are drag with no muffler. No jetting to the carbs...
Have anyone some ideea? First thing in my mind was needle jet. But to rich or to lean?