Hi I have managed to rectify it. Nozzle length and bucking was fine. I swapped out the sector gear (as one inside was somehow 3 mm wider in diameter and kept rubbing on trigger) added a sector delay to sector gear, corrected AOE and is now shooting fine. I measured the original nozzle to the after market one and they are both 19.7mm. not sure which of the above was the help but it's now fine. Gun is shooting around 320fps , I use ASG devil 0.28g bbs , ROF is around 17rps
Only thing I could think of was maybe the tappet plate had misaligned somehow causing nozzle not to return properly, but on the other hand I don't understand how it fed flawlessly on semi.