1999 ram 5.9 backfires

I recently repaired a leaking intake manifold plenum gasket, and at the same time replaced the plugs. After putting it back together, the engine now has a irregular exhaust backfire. The backfire seems to occur less frequently the longer the engine runs.

The vacuum holds at a steady 16-17 pounds, no sign of a major vacuum leak. New plug wires, rotor and cap have been installed. No codes set.

The owner tells me that the truck would occasionally backfire once shortly after starting (cold engine), but usually only one time.

The one thing I didn't do, as recommended by the tsb, was to run a combustion chamber cleaner.

I believe a rich fuel condition may cause an exhaust backfire type sound when the unburnt fuel enters the cat. Could a damaged injector cause this?

I intend to check the timing, but I don't see know that would have been changed.

Any ideas??

Thanks

loose timing chain
have dealer reset the timing
throttle body clean?

Thanks Kevin. I found the problem. My "assistant" mis-labeled the spark plug wires. Found the mix up & it runs fine now. A good reminder to look for the basic things first.