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I have owned a 71 royal scents new... 44,000 miles sat in my shop for years...(hence the miles) been haveing problems with it for 5 or so years..... Ok so I can drive it for about 20 to 25 minutes and it runs great... then she starts back firing and popping though the carb.....
what I have done.. New distributor, wires, plugs, fuel pump, lines, gas tank,changed over to electronic icnition (I know I spelled it wrong)
It has back fired so bad I have blown mufflers off the car....
So my next step is to have heads done to see if there is a week spring...
Do I have a bad lifter?
Thinking is my intake cracked under carb?
Any help would be great... I am not dumb but have been banging my head on the wall.....
P.S.The motor was built back in 71 with flat top pistons and bigger cam... have all the paper work that was done to it...
Here is her pic so you can see she is not something that has been in a field she has been in a heated shop.....
Have you set your timing with a timing light and your idle speed with a vacuum gauge and tachometer? Just make sure you have the basics taken care of before doing anything else.
Like Deja vu. At least about the back fire and ballooning the mufflers. I had a Crane electronic unit in my points distributor. It acted up going to Norwalk. When I left off the gas it would load up and back fire. I took the unit out but never found anything for sure. I did electric tape the wire better and reinstall it, ran fine, then I fixed the mufflers. It's been 3 years, no problem. So, I would be looking at the distributor. Mine only did it when I let off of the gas. Nice car.
It could be a lot of things from an overly rich carb to valves .
Typically, backfiring through the carb indicates an overly lean condition. Backfiring out the exhaust is overly rich. I'd suggest that before disassembling or replacing anything else, try some troubleshooting. The most effective test would be to temporarily install a fuel pressure gauge at the carb inlet and watch what happens to fuel pressure when the backfiring starts. Does pressure go down at that point? If so, I'd suspect that the fuel sock in the tank is getting clogged with crud in the tank, causing the engine to become fuel starved. Once the engine dies or is shut off, suction stops and the crud drops back into the tank, causing the cycle to repeat the next time you start the car.
Did you do anything to the distributor before this happened the first time? Do you still have points? When this happened to my 1971 Royale convertible out of the blue, I was driving on the turnpike at 70 mph +/- for an hour. I let of the gas going on to the exit and (if I recall right) when I gave it the gas it back fired. It reminded me of in the 1960s guys would in a stick shift car shut the ignition off while keeping their foot slightly on the gas. Load unburned gas in the cylinders/exhaust then hit the key and back fire. I had a friend who's father very rarely let him have the only family car. It was a 4 door 1950 Plymouth flathead 6, 3 on the tree. We talked him into doing this going down a large hill. Boom, he blew the muffler wide open. His father was very cheap and he never let him have the car again. This loading up effect was the first thing that came to my mind. The second time it split the mufflers. Damage done, I drove 100 miles home, car idled fine, accelerate fine but after letting off give gas, boom. I had put a Crane electronic unit with a rev limiter. I took it out, and looked at it. I never found anything for sure. I reinstall it and 3 years later it's been trouble free. I always felt it was in the advance or the rev limiter or placement issue with the Crane unit. During the time this happened I went to the Rootstown swap meet. A guy had a set of mufflers that had cut off pieces of exhaust pipe still in them that he tried to get one side of and bent the end on one. Dynaflow I think. I look at them, $20 for the pair he said. I put the one down and started leaving, $10 he said, I said, I don't really want to be carting them around for the rest of the day. He says, $5 and you can pick them up on the way out. Sold, their on my car still.
At least about the back fire and ballooning the mufflers. I had a Crane electronic unit in my points distributor. It acted up going to Norwalk. When I left off the gas it would load up and back fire. I took the unit out but never found anything for sure. I did electric tape the wire better and reinstall it, ran fine, then I fixed the mufflers. 
