Roy wrote:
The above seems reasonable to me but it does not explain why the engine is
being starved of fuel when the the fuel level in the tank drops. I do not
recall having any such fuel starvation problem before the primer button
became torn. So I don't really know what's going on.
The later series of Briggs&Stratton engines (I think they call these
the Quantum series) use manifold pressure to wiggle a diaphragm that
pumps fuel up to a bowl under the fuel jet. When the bowl is
full, it overflows back into the tank. I think the primer
just wiggles the diaphragm the same way, but I haven't had one
of these carbs apart yet. If the broken primer button is
leaking some of the pressure pulsation away, the pump action
may be reduced, leading to fuel starvation when the tank
is low.

Jon