The mower has some years on it.
Recently is stalls after startup. New plug, changed fuel.
I noticed it is backfiring through the muffler.
If I move the throttle half way between run and choke it seems to run but misses.
Any ideas?

How old? sounds like its time for a good carb rebuild..... tear it down, soak it in carb cleaner and blow it out through any and all holes with compressed air and use a rebuild kit to rebuild it.