"Derek" wrote in message
...

"Bob Hobden" wrote in message
...

"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

We have a mower with the 4Hp B&S which also hunts I found using the Shell
Optimax cures the problem with no adjustment needed in this setup there is
no throttle adjustment ( a retrograde step IMO ) - the carb is mounted on
the tank precluding fitting a filter inline.
Derek


I've found the cause of the problem. There are two springs near the carb.
One applies the tension in the throttle cable and there is a second very
small weak spring, which seems to perform some sort of stabilisation - this
was broken. Until I can get a replacement spring I've hung a bit of wire
weighted with a little washer from the attachment point of the spring - this
seems to supply just enough tension to stabilise the engine speed. Too slack
and it runs at an erratic speed but too tight and it revs like it's going to
take off!

David.