In article ,
noname wrote:

I have a fair;y new mower with a Briggs and Stratton engine. The mower has
always started reliably but this year it started sputtering right after
starting it.

I checked the gas cap, fresh gas, clear the air filter, and cleaned the carb
with carb cleaner and Chemtool B12. Now the thing runs for 10 to 15 minutes
like it is brand new and then starts sputtering. I have tried this over and
over and it always starts sputtering at that point.

Really puzzled, any ideas?

Try this... next time it acts up, crack loose the fuel cap and see if it
doesn't magically clear up.

If so, find and correct the cap's vent blockage.

If that doesn't do it, I'd suspect a thermally induced issue... like
maybe a coil or condenser.

Could it be overheating? Are the cooling air passages under the baffles,
and between fins packed full of grass or some other spooge... like maybe
a mouse nest?

Erik