Ok...so I have googled this and there are quite a few posts on this forum but this is a strange one, my kids bike was running fine Friday night, no complaints from the lad. Last night however he was complaining that the bike was bouncing, he could feel it in his chest and head, so I thought I would take it for a ride. The bike accelerates normally, when you get up to 40-50 MPH and let off the throttle the bike appears as if it is bouncing up and down (think small dabs of the brake pedal). I tried applying the front brake and that did not affect it, then I tried with the rear brake and that showed no change either. The three wheel weights look to be intact and in good condition, the brakes have about 2K on them and all front end bolts have been checked for issues. The only thing that he complained about on Friday was the suspension at the back was a bit harsh but I put that down to him having a passenger as it did not seem bad when I took it. Tonight I am going to take it out, take it up to speed and kill the engine to see if it still there (ruling out engine vibration as a culprit). Looking to see if anyone has any other suggestions that I may try...tires look in good condition and the wheels do not seem to have any dings/dent.