This seems like timing but I don't know much. I had a car with a bad dwell setting that did this.

Is the dist advance working OK?I frankly would check wires sounds like cross wires nad I would give it more timing I had a similar issue a number of years ago and all it was the engine needed more total timing . Forget the Mopar perf total of 36 Give the engine what it needs I have had engines that needed as little as 30 degree's and some with 45

If absolutely nothing was changed (& it sounds like that is the case) but the heads then I'd think that the valley pan gaskets ain't sealing in spite of being changed numerous times. Not intending to bash 440source but they do sell heads as blems on rare occaisions that have porosity problems & maybe one slipped by the inspector(s) but I'm thinking the valley pan. What I'd do is block up the metering rods (all 4) (carter/brock) or a gutted PV's (Holley & both of em) as you said dual fours right?. take her out again & see if being on the power circuit makes it straightens out. You'll have an answer pretty quick

You're getting a lot of suggestions and most are good. So I'll toss mine in. Did he try a different coil?

Just jumped in here so--Do we know the history of the engine? Like maybe the heads were milled to straighten them? Then if so possible the intake was also milled to fit the heads correctly? If so maybe they are not fitting right and this could be your issue?? Just throwing it out there. Kevin

I agree lots of good suggestions. He lives out of state so I've only been able to help him two times since the heads have been put on.

I forgot where were the motor came from but he did rebuild it. The stock heads were replaced with some closed chamber iron heads I ported for him and had .010 milled off just clean them up(grinder got away from me while polishing the chambers). Not long after that he bought a used 2x4 tunnel ram with 660 holleys. That combo had been running for about 7 months without issue even with a trip out of state to come see me.

Last fall he bought the heads and swapped them out and the popping starting.

No oil in the intake ports. He trashed the stock valley pan gasket and used an aftermarket cover that doesn't go between the heads and intake. The intake gaskets now are some expensive fel pro ones. I asked him last night if there was a good mark where the intake got tightened to the heads and he assured me there was.

He has ran through the spark plug wires a few times and last time I was over there I did as well along with taking out the dist, brain box and coil. Set all that up on a bench and tested it. Granted I know sparking in normal air is different that compressed air but figured it was good enough. Oh and never let an old man with a twisted sense of humor touch the dist. with you are holding a plug wire. He dang near killed me.

The shop told him it would be a week or so to test the heads.

No he didn't mess with the timing chain while swapping the heads but just before he pulled the heads off again he pulled the front of the motor off to make sure it hadn't jumped a tooth which it hasn't.

Lash was set using feeler gauges. Normally sets it at .018 but tried .012 and .024 with no difference made.

I would say try a different brand of plugs I had a buddy build a brand new engine and this happened and it turns out some of the brand new plugs were bad. Switched them out and ran clean up to 7500rpm's. Also check the wires at night in the dark and see if they are arcing anywhere,I had that happen and the wires were not that old but near the headers there was lots of heat....Also I would check the carbs out they could be lean if it's popping cause it's lean you probably should be able to see the headers glow at night also if it has headers...

just curious but did he swap out the springs locks and retainers?
I know they have 7degree locks and im pretty sure the springs are only good for .510 lift. I don't know if that would cause popping but it might be something else to look into that cold cause damage later

This is just a thought, could the shaft between the distributor and oil pump be off a tooth and mess with the timing?

If you changed nothing but the heads you must do a compression check, sounds like valves not seated to the seats. Like a burnt valve would sound like.

MLR426

Change the coil for another one that is known to be good.

I would take a look at the valve keepers. We had a very similar thing happen back in 1990 with a fresh rebuilt 454. Engine would start popping like a slight misfire at 3/4 throttle. We changed plugs, wires, nothing improved. Our engine builder said to head down the road - we had to leave for a trip to AZ. Found out what was wrong when we were almost to Caro, IL. on Sat. night as we dropped a valve into a cylinder. The Roush guy that did our heads said a valve keeper failed & it was bad from the outset. There, now you don't have to AMHIK!

Ignition wiring from ignition switch thru connector thru bulkhead thru to coil. Make certain that is wired to positive side of coil. Check entire path with meter or test light while helper wiggles entire path of circuit. Basically ensuring that you are receiving proper consistent voltage to coil.

Feel any miss at higher rpm or during acceleration under load? Sounds like not doing well there either.

Try running a wire directly from battery to positive on coil as a short cut.

Intermediate shaft is in the proper location. He is on his third coil. 2nd got ruined by doing what sxrxrnr suggested, running 12v to the coil but he forget to disconnect it over night.

Relayed the message about the keepers. Will post back when he gets it all put back together. If you hear a loud yell then it still pops after it gets back together.

Thanks again all.

What about the push rods did he check them?

Pushrods looked fine. Machine shop said heads looked fine. He decided to put the old cast iron heads back on and surprise the popping is gone. Not one freakin clue as to how or why. He was all excited to have it running again and that's good enough for me.

Thank you all very much for the replies and suggestions.

Silly question, or maybe not??
Was there a mismatch between the exhaust ports and header / manifold creating a sealing issue??? just had a car through the shop with mismatched Copper header gaskets, they were leaking everywhere, and the exhaust was backfiring randomly with no reason why. New properly sized gaskets, backfiring GONE. Just a SILLY QUESTION

My question is when does it pop in the exh ? If it does it on decell then check the header gaskets as if they leak a little it can suck in air and the mixture is richer on decell since the throttle is slammed shut basically and it still has fuel going in but the air is cut off and if it pulls air in the exh system from a small leak it will pop in the exh. Kinda like what and air pump will do if the diverter valve goes bad. But thats if it pops on decell. Ron