So as of this afternoon my 640 ran like silk then I was giving the kids a ride down the street 1/2 to 3/4 throttle and all the sudden it developed a sputter and a click. It will still fire up easy as ever and will throttle up but now it vibrates a little more than normal runs a little rough (like a miss) and has a clicking at the front left corner of the motor. It sounds kinda like an exhaust leak but to have a sudden exhaust leak that caused a miss in the motor seems strange and unlikely. My initial gut reaction was a sticking lifter and a valve not functioning but I quickly learned that they have solid lifters so that seems unlikely to impossible. If I pull the plug wires while running it?s clear all cylinders are still firing. Maybe the rockers suddenly fell horribly out of spec? Broken valve? Broken rocker? Broken push rod? Any knowledge or lived experience on the subject would be Helpful. Thanks