You can tell from the cam chain that this is a VF500 engine. All the other first-gen V4s had Hy-Vo cam chains. This one is roller. What you're looking at is a mistimed cam. It's very easy to get the cams out of time on a Honda V4 because the marks are difficult to use. Honda actually taught us in factory school to use a Sharpie. The symptom of having even one of the four cams slightly out of time is the engine runs fine, maybe a little flat, but fine, all over, idle to redline. When cold. But when hot, the idle will not stay put. It goes all over. Stalling is common.