Lawnmower not running evenly

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goosey

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Hi, Dont know if anyone can help or suggest anything... I am not particularly mechanically minded so any advice would be very gratefuly received!
About a month ago I bought a brand new Mountfield SP164 lawnmower, its worked fine for the first 3 mowings but today for some reason it will not run evenly. So it sounds like its revving up and down (brm brm brm....that sort of noise!)
I can see a vertical lever on the side of the machine that I assume is the throttle maybe, if I move it the engine will speed up, this lever is moving back and forth of its own accord creating the revving up and down. The spring and cables that are attached to it appear to be ok as far as I can tell.
The only advice in handbook about engine not running evenly is make sure the fuel is not over a month old. Well its probably been in there nearly a month but I really don't think this is the problem.
Anybody got any ideas please?
Many thanks.
 
That is characteristic of an engine 'hunting' when it is not getting enough fuel, probably due to a partially blocked jet in the carburettor. Hopefully you used clean fuel and it's a simple job to clean the carb but you need to know what you're doing. If you are confident of having a go there are loads of YouTube videos to help.
 
Yes it was a brand new petrol can and clean fuel so I don't know how anything would get in there to block anything. Not sure I'm confident to do it myself and I guess if I messed it up that would be the end of my warranty? But thanks anyways..
 
Hi Goosey.

As you have surmised, that lever moving back and forth is the throttle control which is activated by the engine speed so that it maintains a constant speed regardless of load.

There are various mechanisms used to control that lever - sometimes the stiff wire link disappears in to the engine casing and is controlled from something internal, and other times it's from a curved 'flap' which sits close to the fan at the top of the engine - the faster the engine spins, the further the paddle (which is in the air flow) moves.

Your symptoms are usually caused by one of two things, both outlined in posts above. One is a blocked carb jet which can affect the engine speed. Some carbs have two jets, one for 'idle' (low speeds) and one for 'running' (high). However, I suspect that these later carbs may only have one - effectively a high-speed jet - since the engines are only designed to run at a single speed, and not idle. But I could be wrong...

I would suggest that if you can hold that lever steady, in the almost fully open position which sounds like its 'normal' running speed (ie don't over-rev it), and the engine runs perfectly smoothly like this, then I think it's unlikely to be a blocked jet, tho' it could be. As you say, it's a new mower, fresh petrol, and one fill - the chances of dirt being there is remote (but leaving petrol sitting in a carb over winter is a real no-no as gums and stuff form on the carb surfaces, and this will cause problems...).

That leaves the speed governor system as being the most likely cause, I think.

Yes, it's very unlikely that the spring has weakened in such a short time, but that's not to say that a wee link somewhere hasn't failed or loosened.

Where did you buy this mower? If a local dealer, take it back there under warranty. If on-line, contact them and find out how their after-sales warranty works.

Worst case is that it was a bit of dirt of grit in the carb so you end up having to pay for the labour to clean it out, but you say you aren't up to this job yourself anyway?
 
Those mountfields have a plastic carb, horrible thing, couldn't even find a idle screw on the one last week,probably hidden under some plastic plug.:(

You can reset governor position on B&S junk.
 
KIAB said:
See this this thread: https://community.screwfix.com/threads/how-to-adjust-idle-and-or-mixture-on-mountfield-mower.199021/

The spring loose it's spring, replaced a spring on a Mountfield last week,identical problem.
Click to expand...

It surely can't be due to the normal fatigue as it's only a month old, so if it is the spring (or a link) then it should be sorted under its warranty.

(Although fairly straight-forward to replace, these springs and links can be bewildering to work on. Last year my in-law's mountfield mower wouldn't run properly and m-i-l told me her gardener offered to sort for her, for £25. It still didn't run properly so I reluctantly had a look. He'd replaced the spark plug with one almost twice as long, and forced the washer half-way down the threaded shaft to prevent it going in too far, and he got the spring fitted the wrong way around... Working out that the spring was wrong and the correct way to fit it was, tho', a real head-scratcher.)
 
Devil's Advocate said:
Hi Goosey.

As you have surmised, that lever moving back and forth is the throttle control which is activated by the engine speed so that it maintains a constant speed regardless of load.

There are various mechanisms used to control that lever - sometimes the stiff wire link disappears in to the engine casing and is controlled from something internal, and other times it's from a curved 'flap' which sits close to the fan at the top of the engine - the faster the engine spins, the further the paddle (which is in the air flow) moves.

Your symptoms are usually caused by one of two things, both outlined in posts above. One is a blocked carb jet which can affect the engine speed. Some carbs have two jets, one for 'idle' (low speeds) and one for 'running' (high). However, I suspect that these later carbs may only have one - effectively a high-speed jet - since the engines are only designed to run at a single speed, and not idle. But I could be wrong...

I would suggest that if you can hold that lever steady, in the almost fully open position which sounds like its 'normal' running speed (ie don't over-rev it), and the engine runs perfectly smoothly like this, then I think it's unlikely to be a blocked jet, tho' it could be. As you say, it's a new mower, fresh petrol, and one fill - the chances of dirt being there is remote (but leaving petrol sitting in a carb over winter is a real no-no as gums and stuff form on the carb surfaces, and this will cause problems...).

That leaves the speed governor system as being the most likely cause, I think.

Yes, it's very unlikely that the spring has weakened in such a short time, but that's not to say that a wee link somewhere hasn't failed or loosened.

Where did you buy this mower? If a local dealer, take it back there under warranty. If on-line, contact them and find out how their after-sales warranty works.

Worst case is that it was a bit of dirt of grit in the carb so you end up having to pay for the labour to clean it out, but you say you aren't up to this job yourself anyway?
Click to expand...
 
As sated earlier, the engine is running on a weak fuel air mixture. I have a Mountfield mower, made in China, carp. Every year I have to clean the carb despite using clean fuel. Their is no fuel filter in the line. As this is new, and you assure us that clean fuel has been used, I suggest as service call or return to supplier for a fix. It is new and within G tee. Maybe suggest a fuel filter is fitted.
 
Bought the mower online from Screwfix...yes will just have to ring up re warranty. The whole reason for buying a new mower was so this sort of hassel doesnt happen...hey ho. Thanks all..
 
Sod's law dictates this will happen on occasion.

It really comes down now to how well the situation is handled. Have to say I have found SF to be pretty good in the rare occasion I've had an issue.

Keep us posted :)
 
Going to add to this thread, since fuel problems has been mentioned - try Aspen fuel for garden machinery and especially for boat engines.
It has a 3 year shelf life, won’t absorb water and can be left inside the carbs without causing problems.
Particularly good for hedge trimmers, strimmers etc because no real smoke or noticeable fumes.
Can save you maintainence costs as main problem with engines is fuel and water left in the carb.
You can buy Aspen for 4 stroke and 2 stroke engines. Expensive, but well worth it for machinery that really shouldn’t use much fuel over a year anyhow.
 
So Screwfix were v helpful and said I can return it (luckily have one close by) and they will replace with a new one, though I don't relish trying to fit it in my small car and assemble a new one again. I have to return it the box but when I said I don't have it anymore they have waived that bit but I have to make sure there is no fuel in it...so next question what is the easiest way to empty the fuel tank? or should I just try and get a friend to see if its a simple fix...but then does that affect my warranty?
Thanks Lucy
 
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