Lawnmower problem
 

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.

David.

Lawnmower problem
 

"David (Normandy)" wrote in message
...
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.

David.



Da..stab..rk

Is there a filter in the fuel line? Clogged?

Mike


--
The Royal Naval Electrical Branch Association.
'THE' Association if you served in the Electrical Branch of the Royal Navy
Reunion Bournemouth August/September 2007
www.rneba.org.uk
"Navy Days" Portsmouth 25th - 27th July 2008. RN Shipmates will have a Stand

Lawnmower problem
 

"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!

Look for a slack spring in the governor mechanism; the throttle only
controls the speed indirectly, by telling the governor what's wanted.


Gary Woods AKA K2AHC- PGP key on request, or at home.earthlink.net/~garygarlic
Zone 5/6 in upstate New York, 1420' elevation. NY WO G

Lawnmower problem
 

"David (Normandy)" wrote in message
...
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.

Also *DON'T* overfill with oil as too much will give a wave effect inside
the case and the governor mech will fluctuate.

Also check the pinch bolt on the governor arm hasn't loosened off.

Loose spring on the float bowl float to the needle valve will make a
difference too.

Lawnmower problem
 

On 6 Aug, 19:04, "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.

There is a sort of metal flap, acted upon by the draught from the
flywheel, which operates the throttle butterfly. There should be a
spring biasing it in one direction, making a very crude constant revs
governor. Oh, and a floppy metal rod to the carburettor. The hand
throttle cable in effect sets the end point for this gadget. Sounds
like something wrong with that lot somewhere

Equally, the choke is operated by the last motion of the hand throttle
cable. Can't recall the details, I think it's a sort of cam. Again a
spring is involved to force the choke device to move against the
control. Something may have dropped off there.

If you know the model of B&S you can probably download drawings from
the web site.

Lawnmower problem
 

"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

Lawnmower problem
 

-
wrote in message
oups.com...
On 6 Aug, 19:04, "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.

There is a sort of metal flap, acted upon by the draught from the
flywheel, which operates the throttle butterfly. There should be a
spring biasing it in one direction, making a very crude constant revs
governor. Oh, and a floppy metal rod to the carburettor. The hand
throttle cable in effect sets the end point for this gadget. Sounds
like something wrong with that lot somewhere


My mower is prone having grass cuttings building up on the 'metal flap',
gives similar symptoms.
Rog

Equally, the choke is operated by the last motion of the hand throttle
cable. Can't recall the details, I think it's a sort of cam. Again a
spring is involved to force the choke device to move against the
control. Something may have dropped off there.

If you know the model of B&S you can probably download drawings from
the web site.

Lawnmower problem
 

On Mon, 06 Aug 2007 14:30:01 -0700, wrote:

The engine keeps revving fast then slow repeatedly every second or two.

Hunting, could be a governor problem but more likely fuel. Other have
mentioned fuel lines a float chambers but if this is a fairly recent
engine it won't have them. The carb and fuel tank are closely integrated,
and the suction from the inlet manifold lifts the fuel into chambers in
the top of the tank separated from the carburetor body with a reinforced
rubber gasket that has various flap valves and a pump diaphragm.

My B&S started hunting and one of these flap valves had got itself
slightly out of position and down rather over its orfice. Carefully
putting the diaphragm back into the right place, fraction of a mm out,
cure the hunting.

If you know the model of B&S you can probably download drawings from
the web site.

The B&S site also shows you where to find the engine/model/series numbers
so you can identify your engine and get the right manual. Though that'll
really only give you an exploded parts list and set up instructions,
rather than bolt by screw maintenance instructions.

--
Cheers
Dave. pam is missing e-mail

Lawnmower problem
 

"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

Lawnmower problem
 

"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?


Yes the Machine needs a new carburettor diaphragm. its just a small piece of
rubber and they need replacing periodically.

Mike

Lawnmower problem
 

"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.