Heavy backfiring at low speeds

I donít know if this should go under Cars & Minivan or Trucks so Iíll put it under both, as this is a small motor home.
I have family in New Orleans so was anxious to get there after Katrina
and took off knowing that I had a deal hole on# 6 (burned valve). Iím driving a 1977 Concord motor home with a dodge 360-3 engine. I put in new plugs and a new carburetor before leaving.
It didnít have all the power I would have liked but it ran OK and I made it there. I headed home (Tigard Oregon) after a couple of months and got as far as Phoenix Arizona. I donít remember if there was a pop or loud noise of any sort but suddenly I could barely make 20 mph. I got towed into Phoenix where I was told that #1 and 3 cylinders were bypassing gas (raw gas squirting out of carb). He pulled the heads and we swapped for a rebuilt set. He seemed to do a good job and the engine ran fine. After leaving Phoenix I was able to make hills at 60 that would have been 45 before the heads were replaced.
I made about 800 miles and then everything went to hell. I could barely get it started and when I did, it sounded like it was a shooting gallery. It was backfiring worse than Iíve ever seen. This started about 20 miles from a city and I pushed on only to find that once I got above 45 or 50 mph it seemed to settle down and ran well enough that I decide to carry on because I really wanted to get home.
I finally made it and was glad to walk away from it. But now I need to get it fixed because I want to go back and work with Habitat for Humanity to help some of those people who have lost so much. But I donít have a clue of where to start. I am thinking about shot gunning in a new distributor to see if that helps, but that is only a guess. I would really appreciate some guidance.
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Issy wrote:

Sounds like excessively lean mixture which can cause the backfires you are seeing and also burn valves too. A burnt intake valve can cause bad backfiring too.
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It could be anything, but given your story and symptoms my guess would be the timing chain has jumped a tooth.
Newly rebuilt heads bolted onto a worn out bottom end is frequently a recipe for trouble. You didn't mention how many miles were on the engine before the first breakdown, but somehow I'd imagine on a '77 it was a lot. With the heads already off, how much extra would it have cost to hone the cylinders, re-ring it and put in new bearings, seals & timing gear & chain? Perhaps a new cam while we're in there. Probably double the original job but if you had done so, you would essentially have a brand new motor except for the carbeurator.
It is also possible the "rebuilt" heads weren't fully rebuilt. Some rebuilders will simply grind the valves & seats and put it back together with all the original hardware (valves, springs, lifters, push rods,rocker arms and keepers). I cannot imagine anyone putting newly rebuilt heads on and not replacing the springs and lifters, but I'm sure it happens.
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Not likely a timing chain, he said he has a burnt valve and they only get worse.
snipped-for-privacy@dodgecity.cc wrote:

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wrote:

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Check for lean mixture as it would cause the backfiring and the burnt valve to begin with.
CBHVAC wrote:

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