New some advice

Could be an intake valve hung open, a cam lobe going/ gone flat, bent/ stuck valve/ broken spring.

Remember you changed heads. If someone did a valve job and "sunk" some of the valves without adjusting stem height, you could be "hanging one" open

Run a leakdown/ compression check

Determine if you can WHAT cylinder it is by doing this:

"Rig" a way of grounding the spark at each plug. One way to do this is to pull the dist. cap boots up, and run small brads/ nails down beside the wires. Then use a clip lead/ probe to ground one plug at a time. Look at an idle for a "weak" cylinder ---compare RPM, and rev the engine with each one grounded, looking for the backfire. IF you find the one that "changes" when grounded, you can concentrate on that cylinder.

MAKE SURE you did not switch two plug wires, or that you have a bad wire, or cracked spark plug.