CharlieS
Gold Member
On a Craftsman riding mower. It always starts fairly well hot or cold, But when warmed up with no load on the engine, it backfires and pops back through the exaust. Throw the mower and deck in gear and mow with it the backfire goes away and it runs perfectly. Stop and take everyting out of gear, backfire returns. Also theres no difference at full throttle or idle.
Compression is good, 110 psi. plug while not new is clean and a bit darker brown than the normal tan one usually associates with a properly burning spark plug. fuel is fresh.
I've thought about the possibility of a carboned up piston, possibly a burned or stretched exaust valve, or possibly ignition/timing or carb issues BUT All those issues should result in it running worse, not better, when putting the engine under a load.
My current hypothesis is that the carb's on the rich side thus dumping a bit of unburned fuel into the exaust with no load, then of course burning that fuel when the engine is put under a load, but the mower hasnt acted this way until mid summer of this year.
Also a neighbor down the road has an old rider with a 14 or 15 hp briggs that suffers the exact same symptoms.
Anyone got any thoughts?
Compression is good, 110 psi. plug while not new is clean and a bit darker brown than the normal tan one usually associates with a properly burning spark plug. fuel is fresh.
I've thought about the possibility of a carboned up piston, possibly a burned or stretched exaust valve, or possibly ignition/timing or carb issues BUT All those issues should result in it running worse, not better, when putting the engine under a load.
My current hypothesis is that the carb's on the rich side thus dumping a bit of unburned fuel into the exaust with no load, then of course burning that fuel when the engine is put under a load, but the mower hasnt acted this way until mid summer of this year.
Also a neighbor down the road has an old rider with a 14 or 15 hp briggs that suffers the exact same symptoms.
Anyone got any thoughts?
