i have a 1978 kz 650. i just replaced both tires. Since replacing tires, the bike has begun pulsing between 55 and 70 kph. It is a steady pusle i can feel through the handle bars and in the seat (whole bike), that is not noticable under 50kph or over 70kph. Front and rear tires were balanced. Anyone have any ideas what is causing the pulsing? Whats the fix thanks