Motorcycle bogging down after a few minutes of riding?

I have a 1975 Kawasaki KZ400D. The bike runs fine for the first minute or so and then it bogs down. After it bogs down it will only go around 40-50 in third and fourth gear. When I have it running I can feel that the exhaust on the left side of the bike is warm and the right side is luke warm or normal; it is not hot. Is this a gasket problem or a carburetor or something else? The pet is also a replacement that is just a generic switch, could this be the problem? Thanks for any help.