XR400 engine problems

Well, I did it, Im pretty sure I blew up a Honda. I did the impossible. LOL! Anyway, I was haulin down a long flat trail (basically a driveway to our property) and when I shifted to 4th, as soon as i let the clutch go, the bike just started to slow down dramatically so i was like crap! I thought it was out of gas but i looked back and all i could see was white smoke. It still will run (I rode it 5 miles back to the truck cause the truck couldnt get to it). It smoked for a lil while then it quit. I only rode it in 1-3 gears at very low RPM but it seemed like it had all the power and all. It didnt smoke hardly at all on the way back. So im wondering what this might be. Maybe it hit a valve or something. Ive seen stuff smoke when tagging a valve. I wouldnt think it would have this much compression and run like it always has with a blown motor. It still has plenty of oil too. So thats not why its not smoking. Im gonna tear it apart and see what it looks like soon but what do yall think this could be from my description?