i have a carb zr500 and it seems to be runnng off of one cylinder. it starts off fine but won't get past 7000rpm and barely hits 50mph at full throttle. has new cdi box and just replaced piston and rings and it still runs like crap. sometimes it seems to come on both cylinders and pop up to 8000rpm but falls right back down to 7000rpm could it be a coil or something else. i'm stuck.