"Brigitte J." wrote in message ...
pop and back-fire? What do I need to do to correct this problem?

TIA,
Brigitte

Two things come to mind:

1) Spark plug may be fouled:

The end of plug which goes into mower will be rusty/corroded or coated
in oil. (replace with same model spark plug - you may need to set it's
gap)

2) Fuel tank may contain water:

You'll see "bubble-like" globules at the bottom of fuel tank if water
is present. If a small amount(size of dime or smaller), you may be
able to add some isopropyl alcohol to "get rid" of it. (I usually
remove the fuel tank & rinse it out with gas.)