left cylnder cutting out

[blue-banshee]

hello, i just put a set of fmf fatties and power core two on my banshee, and also go a dyno jet kit, the kit gave me 280 mains and new needles witch i have on the forth cilp because of my elavation, anyway it idles really high i got the carbs synced by my yamaha dealer, so it should be good but its not, it runs very good somtimes but other times the left cuts out but if a do a 6 gear full throttle run the left kicks in and then it will run fine for a little while its really gettin anoying on min i have all the power and the next it run on one, it ran varry well on both before i put the pipes and kit in, what did i do rong? thanks