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Be sure that you have the correct carb for the engine. The distributor advance in your car gets the vacuum from a port on the carb, itself, and not raw vacuum from the intake manifold. Backfiring after cranking without the ignition on loads the system with fuel, which ignites when the ignition is turned on and the engine cranked. It can also be caused by fuel leaking into the manifold from a faulty power valve. A backfire can rupture the diaphragm in the valve, so it is good to check your to be sure it is OK after the backfire.