i have a 1998 zr 500 carb and it seems to only be running on 1 cylinder. it starts off fine but only revs up to about 7000rpm and does only about 50 mph. i replaced the cdi box and put new piston and rings in. sometimes it will get up to 8000rpm but fall right back down to 7000rpm. almost like it just came on both cylinders for a split second. could it still be a bad cdi box or maybe a bad coil. i'm stuck.