Hi. I don't know where to begin, So I guess I'll start from the beginning. I own a 1994 ZXR250 that I put together from an $800 ZXR250 I found in my neighbors back yard. Ever since I got it, it's been having one major problem. It hesitates and stutters between 4k-8k RPM, but only when warm. When it's cold, it performs beautifully, not a single trouble in sight, when she gets warm, everything turns to shit. When it stutters, there is little to no acceleration, I have to clutch in, hold the throttle wide open and eventually it breaks through and returns to normal, but after about 5 minutes of riding it starts to stutter again. She used to be running rich, but I turned the pilot screws down to the stock 2.5 turns, I've readjusted to floats to 13mm stock settings. Replaced the spark plugs twice. Cleaned the carbs several times. The previous owner rejetted the bike and replaced the stock jets (122, 125, 125, 122) with bigger jets (125, 128, 128, 125), and strapped on an after market exhaust. I have no idea what could be causing it.