I'm trying to start a freshly rebuilt SBC. It'll pop and snort but won't run. It will occasionally backfire through the carb. I've loosened the valve lash to zero lash to eliminate the backfire. ( I'll adjust the valves once the engine is running). 1. I've tried rotating the dist. 180, 2. I've triple-checked that the plug wires are in the correct firing order. 3. I've checked the timing gears are properly installed. 4. I've got plenty of fuel (fuel squirts down the carb when I open the throttle. What am I missing?