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new 496, everything new. went through cam breakin, driven about 75 miles. car ran prefectly. next day when I started it had a backfire in carb. started engine and ever since have a constant backfire in exhaust. local shop said power valve in holley, replaced it, checked wires no crossed wires, pulled plugs all look good, ran compression all at 160. thankful for any ideas to check next.
 

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I had that happen and it turned out to be bad pick-up coil in HEI
 

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Everything NEW. On Break In run was fine but Back Fired through Carb and Exhaust when Starting the next Day and compression is uniform which should eliminate a Valve problem but never the less always check lash and timing after a Break In - Humm.

Suggest you put a Vacuum gauge on her to determine Valve Problem, Carb Base or Intake Vacuum Leak.

Possible sticking Float Needle or Float Level problem.

Also if one cylinder isn't firing it will dump gas into exhaust and cause back fire through pipes. An Air leak in the collectors will also cause exhaust to Backfire when letting up on the throttle and as mentioned possible intermittent Ignition cause by various reasons - Ignition Sx, Bad plug, Plug Wire. Distributer Trigger, Coil, CD Module ... etc.
 

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I've seen the module go out in my buddy's supercharged Mustang while I was behind him and it went KA-BOOM and left two perfect black circles from the tailpipes on the freeway onramp.
 

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Over tightened rockers perhaps. That's a common problem on a new build. It's easy to misinterpret the adjustment instructions or not rotate the engine properly during the adjustment. If the rockers are too tight, they will leave the valve hanging open.
 

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FWIW, the Holley website says all their carbs since like 1992 have had a check valve so the power valves don't get blown. Maybe you paid someone to replace something that wasn't bad???
 

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I experienced same thing last week... I went to give her some gas and shift into third gear. The engine felt like she was dying. I gave a little gas and she took off and then backfired REAL LOUD! I checked everything under the hood and found that the cap was loose. I tightened everything up and all was better! In my case, I'm sure that the plugs weren't getting spark and then when I hit the gas, it gave enough to set off the raw fuel in the pipes and blam-o! :eek:
Dano:D
 
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