Ld4b early edition manifold

krazykuda said

Try retarding the timing a bit until it goes away...

Check your total timing between 2500-3000 RPM, then see what it is at idle.

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Backfire usually means retarded timing. Also, make sure you have good accel pump shot. Weak shot can also cause backfire.

Check vacuum advance as well if backfire happens at throttle tip- in.