I spent the past few months reuilding a 350 for my 81 K10. New plugs, wires, rotor, cap, rebuilt holley 4160 carb, new cam, rings, lifters, crankshaft, all bearings. I have spark at the plugs, gas in the fuel bowls, valve timing correct, but it won't start. It turns over and tries to fire, but won't. It did run for about 10 seconds but won't stay running. What should I check next?