I just finished a valve adjustment. It started right up sounded ok, then went to 3k rpm and sat there for a minute or so, then the rpm dropped and ramped back up. It started cycling between 3k and almost stalling. I shut it off and looked for vacuum leaks or a disconnected sensor. Didn't see anything so I tried it again, same thing after about 20 seconds. then the check engine light came on. It's dark now but I'm wondering if a vacuum leak would do this or if it's a sensor, could i have mis-adjusted the valves? It was running fine before I messed with it.