"David (Normandy)" wrote...
The engine keeps revving fast then slow repeatedly every second or two.
While it still cuts the grass without problem it makes it's own mind up
how fast the engine goes, so I alternately walk slowly behind it then have
to run after it! The throttle setting has a noticeable, but small effect
sometimes, but only for a moment or two.

Suggestions?

The mower is a "Best Green" with a Briggs and Stratton 4 stroke engine.

It's called "Hunting" and is usually caused by fuel starvation.
Check fuel lines and any filters for dirt but it's probably the carburettor
that has dirt in the float chamber so take it off the machine, take the top
off and clean it out using petrol. Be careful though, it's a precise
instrument, do not damage the float or you will need a new carb. 10 min job.
You can get plastic in-line fuel filters at most motor spares shops so get
one of those and put it in the line between tank and carb.
If you damage the paper gasket between the top and bottom taking it all
apart make/cut a new one using the cardboard of a cereal packet.

--
Regards
Bob Hobden
17mls W. of London.UK