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

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.



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