My 1987 K100RT is continuously burping (rumbling ) in the muffler, indicating that unburned gas is getting in there. It is not loud when doing 65 mph on the HWY, with one exception: going full blast / full throttle at >110 mph, then closing the throttle usually results in a loud explosion. In general good acceleration and power. Cracked or dirty exhaust valve? Valve timing? ignition issue?