Small Engines (Lawn Mowers, etc.): briggs backfiring though muffler, briggs and stratton, lean mixture

Question
Paul working on a briggs and Stratton engine. Model# 31707 Type# 0136-e1 Code# 00100930 Engine backfires when started though muffler loud like a gun going off. some times it will start and some times will not. Adjusted the valves and still does it. Checked the flywheel key and is ok. Removed head and check valves, pushrods and head gasket all looked good. It also has good spark and is getting fuel. Any ideas would be helpful. Thanks bc357

Answer
HI BC357

SUSPECT THE MODEL IS 311707-0136-E1   0HV  SINGLE CYL   ELECT START  

DID YOU ADJUST THE VALVES IN /EX  AT 1/4 PAST TDC ON COMPRESSION STROKE IE( 1/4 DOWN ON THE POWER  STROKE ) THIS ELIMINATES DECOMPRESSOR  

BEFORE REMOVING CYL HEAD  DID YOU CHECK THAT EX VALVE WAS NOT LEAKING / OR BURNED?

ONE WAY TO TEST VALVE AND SEAT SEALING IS TO POUR SOME PETROL INTO EXHAUST  PORT  AND SEE IF IT POURS OUT PAST THE  EX VALVE SEAT AND VALVE -YOU WILL SOON SEE IT

DO THE SAME FOR INTAKE   BOTH MAY NEED  GRINDING TO THEIR SEATS IF BURNED  / LEAKING  

THEN PULL OUT VALVES AND CHECK FOR PITTING / BURNING- ETC

THE OTHER THING COULD BE THE AFTERFIRE SOLENOID MAY NOT BE  CLOSING FULLY OR IS FAULTY  
ESPECIALLY IF UNIT HAS A LONG COAST DOWN TIME AFTER BEEING SHUT DOWN

SOMETIMES POPPING / BACKFIRE CAN BE LEAN MIXTURE  - YOU MENTIONED TIMMING KEY WAS GOOD

ANOTHER THOUGHT   MAGNETRON UNITS CAN PLAY UP  BUT IF YOU HAVE THEM PUT ON A TESTER AT YOUR LOCAL SHOP AND ARE CHECKED WHILE UNDER LOAD AND HAVE SOME HOT AIR BLOWN OVER THEM COULD SIMULATE A SPARK BREAKDOWN - HENCE POPPING / BACKFIRE  BUT WHEN COLD SEEM OK WHITH NO LOAD ON THEM  , HAVE SEEN THIS BEFORE ON A 12.5 B/S  SIDE VALVE ENGINE

CHECK PLUG HEAT RANGE MAY BE TOO HOT , CHECK COMBUSTION CHAMBER FOR ANYTHING SHARP OR SOMETHING LIKE CARBON THAT MAY GLOW AND CAUSE A PRE-IGNITION     STD PLUG IS RC12YC  3/4 INCH REACH

MAKE SURE IGNITION UNITS ARE OK - MAKE SURE GROUNDING WIRES  ARE NOT TOUCHING EARTH SOMWHERE INTERMITTANTLY ON / OFF  CAUSING A BACKFIRE  

CHECK CARBY  COULD BE FLOODING

ANOTHER THOUGHT  CHECK MAGNETRON UNITS  THEY COULD BE BREAKING DOWN HOTAND UNDER LOAD
BUT SEEM OK WHEN COLD  HAVE YOUR LOCAL SHOP TEST THEM ON A TEST MACHINE WHILE BLOWING HOT OVER THE ARMATURE

HAVE SEEN THIS BEFORE ON 12.5 B/S ENGINE


CHEERS PAUL

HAVE A LOOK  AND KEEP ME POSTED