I just bought a '98 YZ400 and took it out for a shakedown day at the family farm. Supposedly has a totally rebuilt engine after the previous owner only drained the oil(didn't refill it) and then went for a ride. Anyway, all was going fine, bike pulled very strong, I stalled it a few times, my bad. Little hard to start but, again, probably just getting used to it after my XR400. Was running great when I stopped for lunch. Came back 3 hours later, kick, kick, kick..... - no luck. Tried towing it behind our trusty XR80. Got it to fire and run for a few seconds, then it stalled when I let off the throttle, no luck after more towing. More kicking revealed a seeming loss of compression. It's was almost impossible to feel TDC, like I had the compression lever pulled in the whole time. I noticed also that the plug cap had come undone but replacing it made no difference. My guess is that it somehow is related to the compression release mechanism as there was no apparent catastrophic event that would indicate a mechanical failure. Any help is welcomed.