big block duster misfire

Same car as in my other posts: '75 duster with a '73 400 big block 4 barrel/727 auto/8.75" combo with a cam (don't know the specs) and new mopar performance electronic ignition distributor with mechanical and
vaccuum advance. Runs smooth since I installed the new distributor, but there is an intermitent kind of "pop" noise that comes from the driver's side cylinder bank occasionally. It is not rythmic so I am thinking a misfire, maybe? The engine runs smooth though, so at first I just chalked the noise up to the crappy, leaky exhaust system. However, the other day I removed said exhaust at the manifolds and couldn't resist the urge to fire it up with no pipes. The noise is very noticeable now and when it does it a small flame spits out of the driver's side exhasut manifold. In addition, after I rev the engine and it is on its way down the rpm range, it pops loudly and blows considerably louder flames out of the manifold. I have been told that this is normal, but I am skeptical. Also, the engine smokes just a little, I'm thinking either worn rings or valves. Could this maybe be linked to the "misfire" I'm hearning? Could I be wrong and the noise could be something besides a misfire? Any advice would be appreciated.
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Just speculating aloud...
Wonder if a leaky exhaust valve would let unburned air/fuel into the header during the compression stroke, then when the valve starts to open on the exhaust stroke, the little bit of unburned fuel/air ignites?

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kinda what I was thinking too. I guess I'll have to pull the heads and find out what condition the valves are in and check the cylinder walls for wear while I'm at it
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An exhaust leak could cause it to pop, I'm assuming it has headers with a 400 conversion their should be a gasket between the head & ex flange Check the plug wires very carefully for burned spots. If you don't have them check the junk yards for some of the GM metal spark plug insulators
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I bought a set of these for my 55 Chevy and it was the best deal I could find. They not only perform well but look nice too. Stainless braided 8.8mm plug wires for a not bad price: http://store.summitracing.com/egnsearch.asp?N00057+%2D59186+115&Ne 0011&NeXID0011

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