On 1/08/2011 9:38 a.m., Roger Mills wrote:

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.

I've checked that, all OK.

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.

Actually the diaphragm is different from that on at least some other
carbs - the YouTube videos I looked at (for an old 3.5 hp engine) have a
carb with two intake tubes, while mine has just one.