On 31/07/2011 8:38 p.m., Chris Hogg wrote:
On Sun, 31 Jul 2011 17:22:00 +1200, Gib
wrote:

My trusty old mower is no longer trusty. It starts OK, and runs for a
couple of circuits of the lawn, then stops dead. The stop is so sudden
it almost seems electrical, but in other respects it's as if the fuel in
the carburettor float chamber has been used up and not replenished - but
of course there is no float chamber, and the fuel system looks
childishly simple. The electrics are so simple - no moving parts except
the magneto - that it's hard to see what could go wrong there either.
Maybe there is something about the fuel system that I don't understand.
I've taken all the fuel-related bits off, and when I feel inspired
I'll take them apart and clean everything. Meanwhile, perhaps someone
with experience can suggest what I should be looking for. The sparkplug
is new, by the way. It does look rather sooty, but not at the business
end (close to the gap).

I have recently had a similar problem with my 10+ yr old B&S mower,
discussed over on uk.rec.gardening in the last few days. It would run
for a minute or so and then stop. Pumping the fuel primer bulb got it
going again for another minute or so. Popular suggestions were the
rubber diaphragm in the fuel pump/carb had perished, or the vent holes
in the fuel filler cap had become blocked. The latter can be checked
by running with the filler cap loose or off altogether. The following
may give you some ideas:

http://www.briggsandstratton.com/eng...he-carburetor/

Current thinking is that my problem was simply a blocked filler cap
vent.

Thanks, Howard Neil has the same idea. No fuel primer bulb on this
engine. It is about 20 years old, and the carb is very simple.