Backfire Through Carburetor

A misfire can cause a backfire from bad spark timing.
Fix that, check ignition timing, then check the floats levels, needles, idle screws and power valves in the carbs. Vac lines are also on the list.
The 600 street avenger should be a much better fit, but a 700 DP in proper tune should still run.

VasiliBrown said:

So I verified timing by taking the distributor out and putting it back in. I put the 750 back on and it fired up fine. What could be wrong with the 600 to cause the backfire?

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Answered in post #2. Check all the adjustable and moving things. A lean backfire can be from a fuel distribution problem. A Holley SW vac secondary carb is likely to have power valve protection, but that could be leaking and causing the wrong mix, too.

How did you check the timing with pulling the distributor, and not with a timing light? You can sometimes get it close while installing it, but the light is needed for verification and tuning in cases like this.