I just replaced the head gasket in my 1992 Civic. Started it up before putting everything back together (just a second since it had no coolant, exhaust wasn't connected, etc.) and it ran fine. But now after putting everything back together and filling the cooling system, it won't start. Turning the key, the starter motor runs, and the engine turns with virtually no resistance - so smooth that at first I thought the starter motor was just spinning without turning the engine.
What's the best way to go about diagnosing the problem? I don't see anything obviously wrong - no visible leaks, etc.