Petrol Mower Advice


I was cutting my lawns today and halfway through the mower started running slowly, approx 1/2 throttle. It is a Mountfield Omega 46cm (18") hand propelled with a Briggs & Stratton 3.5 HP engine. At first I thought it was the petrol running out so I looked in the tank and it was nearly empty. I filled it up and it was still the same. I did all the usual things, clean plug, air filter, set plug gap etc with no effect. If I pull the throttle lever it revs OK. I took the starter lid off to look under there and nothing seemed amiss as far as I could see, the fan blades were intact and the flap that is moved by the air from the fan blades moved easily enough.
There is a block made up of lots of layers of thin metal at the front of the engine that I presume is a coil type thing as the HT lead comes out of it. I noticed that this was touching the big wheel in the middle at one side and not at the other. What is the correct positioning for this bit and is it supposed to be magnetic as it isn't?
Any ideas anyone? Any help greatly appreciated. Failing that anybody know of a good lawnmower service man in the Leeds area?
Cheers
John
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John wrote:

Sounds like the governor has gone weird.
IIRC there is some kind of rotating mass thingy that pulls a lever attached to the throttle

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John wrote:

Have you checked the filter?
Is there water in the fuel? It's a Briggs and Stratton so it will run on low grade fuel without any fuss. If it was a two stroke, the fuel might have become too oily after standing all winter. But that would become obvious as soon as you started it after refuelling.
So if it isn't water or too much oil in the fuel, it must be the timing. But it revs OK when you tickle it so it isn't them. You haven't you hit something like a rock or the kerb, whilst it was running, have you? If not, it is mostl likely the filter.
I'm afraid it just leaves the filter.. unless... one other thing.... there is the faintest chance it might be is ... a blockage in the tank. It could be in the filler cap vent or some debris got into the fuel line. (But how would it rev though?)
It's got to be the filter.
I think it's the filter. What do you think?
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"John" wrote:

You could try http://www.repairfaq.org/samnew/lmfaq.htm
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<snip>
Had a similar problem with a B&S engined Hayter recently.
It was under a year old and Hayter arranged for a dealer (Woods or Horsham) - very good, but a long way from Leeds :=(( to look at it.
He replaced the flap/gasket thing between the tank and the carb to no benefit, but the thing that cured it was...........
A new plug.
I had been taking the old one out and cleaning it, and had almost bought a new plug the weekend before, but I thought can't be the plug can it - it can!
For those who are interested the Hayter machine (and maybe this Mountfield) has no throttle cable or manual choke. the cable from the dead mans handle which you might think would operate the throttle appears to operate a cut off switch for the low tension circuit and applies a brake to the flywheel.
All the chokery and throttlery is done automatically.
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snipped-for-privacy@connectfree.co.uk wrote:
<snip>
Forgot to add, the man from Woods also said to never tip these machines over on their side towards the petrol tank/carb.
If you do, the oil from the sump ends up in the carb!.
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