Just off Idle Backfire

I've got a 390 with Comp Cams 268H cam, 390 P.I. heads (same as 390 GT but not double drilled on the exahust), 9.5 to one pistons, Petronix conversion, Edelbrock 380 3X2 intake with Rochester 2G carbs.

I have recently converted the carbs to true Tri-Power style with the Speedway kit, so I have a new power valve in the center carb and the carbs are caliberated correctly. I have also checked all possible sources of vacuum leaks. Checked the accelerator pumps settings,etc,etc. I had earlier went to a new vacuum advance on the distributor.

The problem is that once in awhile, not often, I get a back fire through the center carb just off Idle when I step on the gas. I've knocked myself out trying to figure this out. Oh, yeah, I had the problem also when I was using the carbs in their original configuration.

My last thought is may be the vacuum advance is adjusted too tight. It is the adjustable type. That would give it a sudden retard ignition setting at low speed.

Do you think this might be it? I am going to toy with it later today. But would like to know if this could be the problem.

I WOULD CHECK THE END CARBORATORS AND MAKE SURE THERE NOT STICKING OPEN A LITTLE SET THE TIMING AT 34 TO 36 DEGREES IF THAT DONT FIX IT I WOULD JET ONE STEP ON THE CENTER CARBORATOR.