I have a 2005 sentra, just replaced head gasket, everything went good, no problems, well maybe a small problem. Now the engine will not start. Did some checking and found out that I only have spart on number 4 cylinder. Double check cam shafts to crankshaft timing and it looks correct (at least by the book). Check all ground wires and all grounds are connected and tight. All electrical plus are good. Its getting gas, just no spark on 3 of the 4 cylinders. Any suggestions????