Help-Can't find source of backfire.

[sprecks]

I'm about to give up.  I have a friends car that I am doing a little work on.  Here's what I started with.
440 that has been sitting about 5 years.  6-pack pistons, 509 purple shaft hydraulic cam, Mopar single plane intake, 850 double pump carb with nitrous, headers, crane adjustable rockers, Mallory unilite distributor, Mallory coil, electric fuel pump with regulator (found set at 9 psi).
Here's what I I have done to it.
Removed the nitrous system, rebuilt the carb, replaced engine wiring harness (old one was a cobbled up mess), installed a new Mancini distributor, new ECU, new Accell plug wires,  new Accell super coil,  new plugs,  new electronic voltage regulator,  adjusted the valves,  reset fuel pressure to 5-1/2 psi. Timing set at 36 deg total.
Here's the problem.
The car will run but misses and backfires out the exhaust. I have tried replacing all of the following one at a time,
a different distributor, different distributor cap, different plug wires,  pulled the intake and resealed it,  different coil,  reset valves.  Nothing made a difference in the way it runs.

What am I missing?  This thing is driving me crazy.

[sprecks]

I also checked compression.  It was between 160 and 165 on all 8 cylinders.

[roadman5312]

Start with the basics.  Firing order.  Double check valve adjustment.   

[Oldschool]

Broken exhaust valve spring? 

[HP2]

I assume you mean a popping out the exhaust while idling and not a backfire on accelleration or decelleration. Did you verify the timing marks on the dampener are accurate and zero was truly zero? I have seen set ups that had backfires through the exhaust due to lean carb mixes. Did you try fattening up the curb idle? Are the exhaust gaskets in good condition? While adjusting valves, did you verify that all the springs were intact and none of them were broken or particular off set in height that might indicated a broken spring or too tight adjustment?

[sprecks]

I assume you mean a popping out the exhaust while idling and not a backfire on accelleration or decelleration. Did you verify the timing marks on the dampener are accurate and zero was truly zero? I have seen set ups that had backfires through the exhaust due to lean carb mixes. Did you try fattening up the curb idle? Are the exhaust gaskets in good condition? While adjusting valves, did you verify that all the springs were intact and none of them were broken or particular off set in height that might indicated a broken spring or too tight adjustment?

Thanks for the input.  I have inspected the valve springs and can't find any that are obviously broken.  I have adjusted and readjusted the valves a half dozen times.  I even put a set of stock non-adjustable rockers on it and had the same condition.  Exhaust gaskets are all good, no obvious leaks. 
I haven't verified the timing marks.  I haven't been able to get the thing to idle below about 1500 rpm without dying.  After running the engine for a couple of minutes I checked the header temps with my point and shoot thermometer.  All of the header pipes vary in temperature.  The right bank had temperatures between 350 and 500 degrees at the head.  The left banks temperatures were considerably lower.  #1 was 220, #3 was 110, #5 was 295 and #7 was 350.  I removed the plug wires from #1 and 3.  Started it up and ran it on 6 cylinders for about a minute.  The backfiring quit.  By the way,  the firing order has been checked and rechecked a hundred times....lol.  1-8-4-3-6-5-7-2 is embedded in my brain.

[sprecks]

After all of the troubleshooting and changing of parts I have done,  I am leaning towards this being a carb problem.  Do you guys think this would be the place to go next?

[jhaag]

Looks to me like number 3 is not firing at all, based on the temps you gave. I would check for, vacuum leaks, bad plug or wire/cap, possibly a flat cam lobe. Yes your carb sounds suspicious as well. (won't idle below 1500=carb issue and or vacuum leak)

[sprecks]

Looks to me like number 3 is not firing at all, based on the temps you gave. I would check for, vacuum leaks, bad plug or wire/cap, possibly a flat cam lobe. Yes your carb sounds suspicious as well. (won't idle below 1500=carb issue and or vacuum leak)

I've tried different plugs and plug wires- no change. I have pulled the lifters and they looked good worn.  When I had the intake off to reseal it, I looked over the cam and it looked great.  Thanks for the input.  I'll look again for vacuum leaks.  I have a couple of carbs on the shelf.  I'll give one of them a try.

[brads70]

Did you replace the power valve (s) ?
I've never had anything good to say about Accel wires. Open the hood while running at night and it looks like Christmas lights on a tree blinking.

[sprecks]

Did you replace the power valve (s) ?
I've never had anything good to say about Accel wires. Open the hood while running at night and it looks like Christmas lights on a tree blinking.

Yep, replaced the power valve.  I'll try turning the lights off and see what the light show looks like.

[quapman]

I had issues like this with a 383 a bunch of years back. The firing order was right, but going the wrong way.

[dodj]

I checked the header temps with my point and shoot thermometer.

Point and shoot thermometers are next to useless unless they cost you about $1000, they just aren't accurate. 
Really sounds a lot like a timing issue to me. You say you set it to 36* total, does it run well at full advance?

[sprecks]

Point and shoot thermometers are next to useless unless they cost you about $1000, they just aren't accurate. 
Really sounds a lot like a timing issue to me. You say you set it to 36* total, does it run well at full advance?

It doesn't run well at any RPM. It randomly backfires at all RPMs.  I believe the backfiring is coming from #1 and #3 cylinders based on the backfiring goes away when I pull the plug wires off of those two cylinders.

[bahamacuda]

Is the factory or a aftermarket tach hooked up? The factory tach on my cuda was bad and it cause issues about the same as yours. Remove tach wire at coil and give her a try.