On 31/07/2011 06:22, Gib Bogle 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).

Well, you've been given a fair number of things to check:
* duff diaphragm in carb
* airlock in fuel system
* ignition cutout operating when it shouldn't, etc.

One other thing to check is the (ultra crude!) auto speed control
mechanism. The throttle cable doesn't open the throttle directly, but
does so via a spring. There's another spring, connected to a vane, which
is trying to shut it. The vane is blown by some fins which rotate with
the flywheel - the faster the rotation, the greater the force. the
actual speed is determined by the equilibrium point between these two
forces - and the idea is the speed remains more or less constant
regardless of load. If bits of grass and other debris get inside the
housing where the vane is, they can interfere with its operation. I'm
not sure whether this would cause your symptoms, but it's easy enough to
eliminate. You just need to remove the top cover of the engine, complete
with recoil starter mechanism, and clean out the housing.

With regard to spares, I get the impression that B&S have used the same
diaphragm in most of their carbs since Adam was a lad, so getting a
replacement shouldn't be a problem.
--
Cheers,
Roger
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