Link to the Bullnose forum page for this ongoing issue.

https://www.ford-trucks.com/forums/1753762-my-460-backfires-a-lot.html#post21223565

Any ideas fellas?
Everything is as it should be, and I will fix my exhaust leaks ASAP.
I feel like I'm running too rich, but no one seems alarmed by the 19in/hg of Vacuum at the Carb.
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Bullnose forum thread

 

check the plugs wires to make one is not leaking voltage to another one .with that vacuum your valves should be sealing .dose it back fire at idle ,or under load ? if it is a holley you probably need to change the power valve ,they get wreaked from back fires .what is your timing set at ? are your plug wire in the correct firing order ?

 

Raw fuel in the exhaust?
Rich condition from the Holley?
Thermactor sending downstream air when it should be farting to atmosphere?
Do some checks on the diverter valve and the TAB and TAD solenoids
Check the float level
Replace the power valve (above info correct about backfires taking out the power valves on carbs)
Have an IM test done on manual mode, get a printout to see just how rich it is running

 

Ok, if fire is coming up through the carb= Ignition Timing is off
If vapors with or without fire is coming up through the carb, then= Cam/Crank timing is off

 

With a carburetor any time you are running along at high engine speed and then close the throttle the manifold vacuum will spike higher and this causes extra fuel to be pulled from the idle circuit. A lot of this passes through the engine and if there is a source of air and a high temperature surface it can ignite and make a bang, blow up the muffler etc. You probably are running on the rich side and you have some air leaks especially up near the engine and that's what is causing the problem. On the carburetor you might want to try cutting back at the idle restrictions by taking off the block and hooking a wire into the orifice. Most of the time I just use a piece of wire from some stranded copper wire. Most of the time the individual strands are about .013 thick. Just hook it through the idle feed restriction on both sides of the primary metering block, put it back together and readjust your idle mixture. What this will do is lean the idle and transition circuits of the carburetor. See if it will tolerate this change without surging at part throttle. If it will you'll also get better mileage.

 

Plugs and wires are new since posting here.
I bought 11mm thick wires and then used split loom to make sure there is no arcing/shorting. I also watched for spark after dark. All good.
It doesn't backfire at idle. Only when coasting/decelerating in gear. Does not backfire in neutral.
I will order a new Power valve. It backfired through the carb a couple of times when I was tuning last year.
Firing order has been triple checked.

 

You are "On Track" for trouble shooting this!!!!

 

Assuming the valves are seating it sounds just like a bad timing chain. Have you put a breaker bar on the crank and checked how long it takes for the rotor to move ?