Mower engine query

 I have a cheap Champion mower which has been excellent for the last 8 years,
it has suddenly started to be very difficult to start.

Symptoms
It wont start at all on choke
on slow it fires but peters out before getting going
after about 15 pulls it finally goes and runs fine until I stop to empty the
grass box, then it will be reluctant to restart (although not as bad as when
cold)
On opening the throttle it revs up and down and is reluctant to settle and
it does similar if it suddenly has longer grass.

It seems to be worse if the fuel tank is not more than half full.

What I know its not
Spark plug, changed that it made no difference,
Air filter, took that off and it made no difference
Safety brake not clearing, checked that and it comes on and off as it
should.

Seems like a fuel problem but I have no manual, so is there a fuel filter
somewhere that could be partially blocked?, or could a partial blockage in
the carb produce these symptoms?

Any other low tech suggestions welcome

--
Charlie, Gardening in Cornwall
Holders of National Collections of Clematis viticella
and Lapageria rosea cvs
http://www.roselandhouse.co.uk  

 On Mon, 04 Jul 2011 14:38:41 +0100, Charlie Pridham wrote:


Small petrol engines can be a bugger to diagnose.

The reving up and down thing may be a clue - there is often a flappy
thing and a spring in the airflow from the blower/magneto on top.  This
is like a speed regulator, and the flappy thing and spring control a
butterfly in the air path through the carburettor, like a second
trottle.  if that breaks/gets clogged with grass you could have those
sort of problems.

Otherwise it is basic petrol engine stuff.  

* Remove and clean the air filter.  Re-oil if it tells you to, otherwise
don't

* replace the spark plug, and gap it correctly, or to 20 thou if unknown.

* Have you got compression? if not the valves may need lapping in, or you
may have a piston ring problem.

* was the spark plug black and caked in carbon?  if so you may need a
decoke and to weaken the mixture

* was the spark plug white, and caked in what looks like limescale?  if
so I bet the plug gap has burned itself to too big a gap, and the mixture
is too weak.

* Have you got a spark at all?  turn it over with the plug lead off and
watch it spark to the frame.  Blue = good spark.  yellow and intermittent
= feeble spark.  You might need to change the points or the capacitor
under the magneto (warning: on some engines removing the magneto without
a "keeper" looses so much flux it will not work again)

Black exhaust smoke when it does start might suggest an over-rich mixture
or too much choke.  No smoke at all might suggest a weak or normal
mixuture and a buggered plug.

--
Bob Harvey OETKB

 

Some of these engines have a rubber like diaphragm in the carburettor that
degrades with age. That can cause the problems you are getting.

Mike

 "Charlie Pridham"  wrote ...

Revving up and down is called "hunting" and is usually down to dirt in the
carburettor which you can usually take apart to clean if you are careful,
some (Honda) have a drain plug on the bottom of the carb too. (well
ventilated area, no naked flames etc.). If there is dirt in the carb it is
also probably in the tank so an in-line petrol filter may be the answer
(from any car spares place) unless you want to drain out the tank and remove
the dirt from the petrol (it will sink to the bottom). (an empty tank that
contained petrol is a bomb so be careful)
Alternatively has it got one of those plastic/rubber petrol primer bulbs, it
may have a small hole in it so it's sucking in air. Had the one on my old
Husqvarna strimmer break up a few weeks ago and had to get a non-original
replacement on e-bay as the dealer couldn't help me.

--
Regards
Bob Hobden
W.of London. UK

 On Mon, 4 Jul 2011 18:51:36 +0100, "'Mike'"



more than anything except the harm caused by a bullying,
confrontational manager who drove his little company into the ground,
couldn't compete in the market and then tried to blame Thatcher.

You always go for the women don't you.
Why are you so frightened of them?