Assistance Needed - Car popping (backfire) through carb

The Charger was driving good until last night. Took it for a drive and first 5 miles or so were fine. I pulled up to a stop sign at a busy road and as I started to accelerator, there was a few pops that came from the carb. Never done this before. I'm not taking WOT acceleration here, just normal acceleration on to a 55 mph road. As I came up to the next corner to turn less than a mile later, the car would not idle and died. It started back up and would keep running as long as I kept the RPMs over 1200-1500 or so. Now there was a routine pop out carb, frequency dependent on RPMs so relatively sure it's related to when firing on a cylinder. Car was in middle of intersection so I started it up, kept RPMs up, and moved car to where I could pull over and shut it off. Of course, I had no tools at all with me since this was a short, quick trip. Initial check only found cylinder one wire on the exhaust manifold but it does have Live Wire plug wires (from Davis United) that have the casing good up to 1400 degrees. Moving that didn't help when I tried to turn it over again.

My mind is running the gambit on this one.

1) Timing off or jumped? Distributor was tight and not movable.
2) Plug fouled or plug wire shorted? Would it behave like this?
3) Exhaust valve stuck? Probably beyond my skill level/knowledge.
4) Push rod bent/broke? Definitely beyond my skill level/knowledge.

Didn't feel confident about trying to dry it 5 miles home so having it towed back Monday. What the heck could have caused this and would behave like this? Like I said first 5 miles or so were normal.

Specs are as follows. Original 400 motor with warm unknown cam. Edelbrock aluminum heads and intake. Carb is Holley 4160. Fuel pressure was reading about 6.5 pounds when I shut it off. Temperature and oil pressure seemed normal. It was not puking any fluids so that was good.

Any assistance or thoughts would be appreciated. Thanks in advance, Todd.

Ok, you most likely found the burned wire (1400 my butt) & a popping like you describe is most likely that plug wire. Timing chain "could" jump, but it would have to be one of those old stock plastic tooth cam gears...otherwise not likely. Good that you checked the distributor. You might take a peek under the distributor cap to see if it or the rotor is burned up bad. Fouled plug and/or fried wire would definitely do what you describe. Hmmm.... maybe an intake valve bent, but that's also rare, so check for that last...with a compression tester in the spark plug hole. If it's a bent valve, one cylinder will have MUCH lower compression, but again, check this last. If you bent a pushrod the engine would clatter LOUD like a jack-hammer, so that's not it or you would have mentioned it. I'm betting on a fried plug wire.

Possibly intake valve spring broke and not closing valve anymore.
Not too hard if it's that.
If you don't have the compressed air setup to hold a valve in place during spring replacement, pull the spark plug on that cyl, feed in a whole bunch of old fashioned white round clothesline, then turn crank manually to squish it all up to the top of the cylinder to keep from dropping the valve in.

Found a problem but not sure if it is the problem. I was testing the plug wire that was laying on the exhaust manifold last night with the ohm meter. It checked fine so I thought I would check the rest. One wire has bugged me since I bought the car. The Davis United distributor on the car sets about 6" higher than stock to clear the valve cover. Problem is that it seems these after market distributor folks assume you put headers on and give you shorter wires to go over the valve cover, not under and around. The number 3 cylinder wire was between the exhaust manifold and valve cover and just barely long enough. When I took it off to test it, it tested fine but the spark plug end snapped off when I put it back on the plug. I also found a chunk of the rubber coating on the wire missing and split about where it was right before the manifold. It may have been arcing but it was behind manifold so could not tell. Question is would a plug wire damaged as above create a pop back through the carb?

It might, but I would think only randomly, unless that cylinder's intake valve is not seating well most of the time, or you have really advanced ignition timing for some reason.
Try pulling the vacuum advance hose and plugging it to see if there's a difference.
If that helps, fix the wire and add tension on the spring for the vacuum advance canister.

Start it in the dark and watch for ignition arch. If there is a bad boot it will show.

Well, getting closer to finding issue. It appears cylinder 3 is the issue, which is the one that had the bad wire. Changing the plug and wires did not help and timing was still correct. Took off the valve cover and disconnected distributor. Still, (quieter) pop back though carb every time 3 cylinder intake valve open. Push rod, rocker arm and valves all seem to be working correctly. Guess not is that intake valve not seating correctly, bent or hanging open a bit in general. I can post a video of this. Any thoughts on how to proceed without tearing head off?

Put Sea-Foam engine treatment in oil, 8oz in crankcase, and the rest in a spray bottle. Squirt soild stream at valve stem and bang valve carefully with copper hammer to free it up.
Wait 12 hours, spray again and bang on valve.
Try it again next morning and see how she runs.

Or Acetone in spray bottle, strong solvent to dissolve varnish.

A buddy came over last night. We believe that an intake valve is not closing all the way, which is causing the popping back through the carb. Makes sense and I want to verify which cylinder it is. I ordered a compression tester from Summit. If we are correct, will the cylinder with bad valve show a major difference, like 30 to 130, or just a minor one, like 120 to 130? Those numbers are just examples. Gauge should be here tomorrow to start testing.

If a valve is hanging open, the difference in compression will be very apparent. Not sealing means very little compression can build. You could also do a cylinder balance test. Pull one plug wire and ground it. Does the popping go away? If not, replace it and try the next one until you find the cylinder that stops the popping. Another way to narrow it down.

Check the voltage at the coil, mine was 11.2 V.

Used a relay to get full battery voltage and backfire under load is gone.

I have found a couple of issues and one may have been related to voltage so you may be on to something there. First issue was spark plug wire on cylinder 3 was in bad shape. The plug "cap" snapped off when I tried to put it back on the spark plug. Also, found a chunk of isolation missing off that same wiring where it was running between the head and exhaust manifold.

Second issues was a power issue. When I went to turn the car over, it was like the battery was disconnected. I found the smaller wire hooked to the positive battery terminal not getting a good contact so cleaned that up and back in business. I did not check voltage to the coil yet so I will do that next.

I have 7 of the 8 plugs out and have checked compression on those cylinders with an OTC gauge. Markings are only every 5 pounds so may not be the most accurate. All cylinders so far have been in the 140-160 range. Only thing I noticed was plug from cylinder 6 coated with carbon and very black, but rest looked like brown. So far, I've been unable to get the plug out of cylinder 8 and the only plug socket I have was a 12 point. I spent about 2-3 hours this weekend rounding the corners on this plug so decided I'm buying a 6 point socket today and will try again tonight. I'm wondering if I could have a blown head gasket between cylinder 6 and 8. The pop through the carb happens even with the distributor total disconnected, just not as loud with no spark. Maybe incorrect, but assuming it's mechanical and not electrical or timing. I can't really test the compression on these two cylinders until I have the plug for cylinder 8 out so kind of stuck at the moment.

Those Edelbrock E-Street heads have a minor issue in my opinion. The only plug I've found with a big enough "shoulder" to have the hex part stick out far enough was an Autolite 85, which was what was in the car when I bought it. I bought NGK XR5 to put in, but the hex part screws in so far I can't get a socket on it. Suggestions on plugs?

Got an answer for the spark plugs. After talking with Edelbrock, they said the recommended plug is the Champion RC12YC. Checking the specs with the Autolite E85 and NGK XR5, there is one major difference. The plugs are suppose to have a 5/8" hex and the other two have 13/16". Explains my issues getting them out. NGK recommended the 7938 (BKR5E V-Power) so probably going with them.

The plugs you are using are short reach. The Edelbrock heads require a .750 long reach plug. The thread is the same obviously.

I recommend the Autolite 3924. Its a long reach plug, one step colder than stock, and uses a 5/8 wrench so you shouldn't have any trouble with the socket.

Good luck,

Steve

I don't like the threads where the answers are not posted, so thought I would close out this one. The major issue is that the lifter (or cam) is not opening the exhaust valve for cylinder number six. Pushrod and rocker not even moving. All other cylinders seem to be working great. Have a short video on phone to show this, but not sure how to upload it. Never liked the top end, low vacuum cam in there, so ordering a Lunati Voodoo 701 cam. Reason for this choice is in another of my threads.

Like most owners here, I did get to experience the enjoyment of previous owner "improvements". In addition to the probably failed lifter (and wrong cam selection), I have found the following three gifts from the previous owner.
1) Spark plug wire fed between OEM HP manifold and aluminum head. Not only did this make the wire brittle enough to break, but was probably causing a slight short on cylinder three since rubber coating was missing a chunk.
2) After market quick release battery connectors were added. I like them but one would think you should clean up the battery wires prior to installing these. After 5 minutes of work, much better connection/voltage.
3) Totally wrong spark plugs. He put the OEM recommended spark plugs in the aftermarket aluminum heads. Edelbrock clearly states for these heads to use a 3/4" reach plug with a 5/8" hex. Apparently the OEM style of 3/8" reach and 13/16" hex was good enough for him. On top of that, he didn't use any anti-seize on the plugs. Having the plugs froze in the cylinder heads and not able to get the socket around them, took about 3+ hours for me to finally get all the plugs out.

One last one not directly related to this issue.
4) Mismatched parts thrown at engine. Earlier on, I contacted Edelbrock because the "off stop light" performance was terrible. A Toyota Prius could have easily out accelerated me. Edelbrock said the E-Street heads are for 0-5500 RPM while the Performer RPM manifold and cam are for 1500-6500 RPM. On top of that was a 600 cfm carb that would starve the engine when doing anything more than accelerating mildly. I swapped the Performer RPM manifold for a Performer to match the head. Swapping to a 750 Holley fixed fuel delivery problems. Last piece is to replace cam.

I'm not sure any or all of these contributed to the lifter failure, but I'm sure none of them helped. One of the many great things about the site is the folks who help out and threads of knowledge base out here. I've never had a lifter failure before and never changed a cam yet. Debating on trying it myself or taking it to a garage, but find out shortly.

Thanks to all for the replies.