Hi, i have gone to a lot of mechanics.., changed the carburator, cdi, rectifier.. i have bought a vt1100c 1996, and the top speed on it was 140 kmph, when im at 4th speed, and change to 5th, my motorcycle decreases the speed, spend A LOT of gas and starts a backfire or something like that, the spark plugs goes to black (carbon), and the bike start to fail, when i run slow like on a hill with a lot of turns, i can make like 130 or 140 miles.., but when im on a highway, i make 70 miles or less.. when i clean the sparks, and run on 120kmph, i can make almost 90 miles.., it´s very sad, i have spent a lot of time and money, my motorcycle has cobra exhausts, k&n air filter, you are my last hope to get a solution, and fix her, my buddies doesnt want to ride with my motorcycle at one side.. thanks in advance!