Just got a 1996 XR400 that will not start. The bike has been sitting a long time so did normal clean, check, carb rebuild, new plug etc. Bike still would not start but would backfire once or twice. Looking further I found a huge gap between rocker/sub-rocker and top of valve stem on left intake. Other valves had normal gaps. Anyone seen this before and if so what was the cause? Stuck valve? Bent rocker? Any advice/comments appreciated before I take the top end apart. Thanks