Briggs & stratton 125cc engine backfires?

Answer 1

may also be faulty valves

Answer 2

Probably lean. Or intake leak--spray WD 40 around inlets, see if idle changes.

Answer 3

False air is entering the engine and causing a misfire. Check carb intake seals.

Answer 4

During excelleration or deexcell?

Answer 5

Mixture is too weak. Dismantle the carb carefully and clean it with aerosol carb cleaner. make sure the jet is clear. Use fuse wire the clear it. You can also buy carb refurb kits cheaply on the net.