On Sat, 30 Jul 2005 20:03:42 +0100, The Natural Philosopher wrote:

It slowed to a stutter and then speeded up then slowed down
and so on.

Mine (with a Briggs a Stratton) will do this after it's been
running a couple of minutes, if the fuel tank has been filled right
up; if I unscrew the petrol cap it's all OK again, so I guess it's
due to a vacuum in the tank.

My B&S developed a similar fault last year, took the tank/carb off to
look for gunge, nothing significant found and it worked fine after
assembly. This year it started doing it again, dismantled again,
noticed that one of the flap valves formed by the thin "rubber" gasket
between the carb and the tank was tucked into the hole it was supposed
to cover and was a little frayed on the edge. On assembly I made sure
that the gasket was positioned so that this flap covered the hole as
much as it could. Works fine now, might be better than it was as well.

These small engines are simple but very cunningly engineered. That
simple gasket together with channels and cavities on both sides and
one spring form the fuel pump, "float" chamber and required valves.
Drive for the pump comes from the vacum in the carb acting against the
spring.

--
Cheers
Dave. pam is missing e-mail