Lawnmower misbehaving

[JoeJoe]

I have one quite similar to this one -

- not sure of exact model. It was bought from B&Q around 7 years ago.

I decided at the time that unlike all the neighbours who take theirs to be serviced regularly (i.e. annually or thereabout), with the purchase price in mind, I would treat mine as all but disposable... All I have done since I bought it was to fill it with petrol when empty, check the oil level maybe once a year or so, and use it (never even cleaned the thing). It has started first time without any problem ever since I bought it, and has done a great job.

Got it out last week for the first time this year, and it was not happy...

Started OK, but was coughing and spluttering and the revs were lower than usual. Also the occasional puff of whitish smoke coming out of the exhaust.

I am the first to admit that I know next to nothing about these thing. My thought was that perhaps the (never been touched before) air filter was blocked, so I looked at it and gave it as best a clean as I could - i.e. a quick brush with my hand, and put it back together.

Continued to use it and after around 10 minutes it suddenly came back to life and was as good as new for the next 20 minutes until I was done...

Took it out yesterday, and the same thing happened again - first unhappy, then happy...

Any thoughts as to what can cause this?

Considering that a new one can be had for around £150, I am happy to play with it - take it apart if needed, etc - but don't want to spend too much money on it.

[Tim+]

A man after my own heart regarding lawn mower care. ;-). I wonder if you have a sticky valve guide causing a valve to hang up intermittently? Some carb cleaner wouldn't hurt.

Tim

[Harry Bloomfield]

JoeJoe formulated on Sunday :

I would suggest checking the carb for dirt maybe blocking a jet. Dirt often gets washed into mower tanks when refuelling them.

[The Natural Philosopher]

Two possibles: you have bad old petrol in it that only runs when its fully warm (add fresh petrol) OR You have too much oil or some wear somewhere and oil is getting where it shouldn't, but once burnt off it's fine.

There's also a chance it has a sticky automatic choke or some choke carb issue.

[Roger Mills]

Is it Mountfield's own engine, like the one you cite, or a Briggs & Stratton?

I know a bit about B&S engines but nothing about Mountfield - except that their reputation isn't that wonderful.

I guess that they all work the same way, though - with a pretty crude speed control device consisting of an engine-blown vane which tries to close the throttle as the speed rises. If any debris gets under the engine cover it can interfere with that and cause erratic running.

Otherwise, with fresh fuel and a clean plug, there's not too much to go wrong.

[JoeJoe]

It is a Briggs & Stratton.

[Jabba]

JoeJoe scribbled...

There's loadsa stuff on Utube

[Dave Liquorice]

Do Mountfield make engines? I have Mountfield branded mower but the engine underneath the labels is a B&S.

All very sloppy and balanced with slack springs.

I think I'd be inclined to run it until almost empty and refill with fresh fuel and see what happens. But if it's done 7 years without an oil change or any TLC I'd give it a good clean around the throttle linkages and springs (but if you take one off make sure you know

*exactly* how it's to go back). The blower and vane. Drain the oil after a cutting session and refill with new clean. Take out the plug clean and regap.

[Davey]

Briggs and Stratton also have a very good Support section on their website, they respond to e-mailed questions. But make sure to check out all possible Help/FAQs first.

- Davey.

[harryagain]

You may need fresh petrol, an easy start component is in most petrols, it evaporates after a while. Best to drain fuel at the end of the year & put in fresh at the start.

If no different. Always start with the spark plug, (Change/clean up). Easiest and most likely thing to go wrong. If you have never changed to oil, there may be accelarated wear allowing oil to contaminate the plug. Check the points, gap and cleanliness. (If magneto driven)

[Muddymike]

I have one quite similar to this one -

Treat the poor thing to some new oil and a new plug, it can only help.

Mike

[Roger Mills]

I believe so. The link cited by the OP says "Powered by a Mountfield

100cc 4-stroke engine". Mountfield also use B&S engines on some models - and then explicitly say so.

I don't know whether they actually manufacture the 'Mountfield' engines, or re-badge someone else's - but they pretty certainly ain't B&S.

[Roger Mills]

One fairly critical item on B&S engines is the diaphragm in the carburettor, which serves as a fuel pump. If in doubt, replace that plus the plug[1], add fresh fuel and make sure that the throttle linkage and speed control vane are free of debris - and you should be away!

Look in the Yellow Pages for local mower repairers if you don't know of any. They should be able to supply the necessary spares. [You won't get them at Halfrauds!]

[Piers]

I had a similar sounding problem with an el cheapo lawn-mower with a B&S engine - sometimes ran fine, sometimes didn't. I ended up taking it apart a couple of times, and the second time I noticed that a pipe had a grove/hole worn into it by constant rubbing against an adjoining sharp metal part. I wrapped some duct tape around the pipe and reassembled. That fixed it for a while.

Might even have been this part:

as I have a record that I bought a replacement (but I never got around to restripping the engine and fitting).

At least, I think this was the lawn-mower and not the strimmer...

I now have a Mountfield with a Honda engine, so I'm expecting better things from that.

[JoeJoe]

Thanks all for your advice.

I think I'll start with the easiest suggestion - drain and fill with fresh petrol. Never had a problem with that, but who knows.

[JoeJoe]

Just a quick follow up:

Finally found the time to replace the petrol in the tank with a fresh one - machines works like a dream!

Many thanks again for all the help.

[Tim+]

How big is the tank or how small is your lawn?? I have to top mine up before ever mowing! ;-)

Tim

[Dave Liquorice]

Thats the first time I've heard a first hand report of "old" petrol giving problems in the UK.

For ours it'll either run out (from full) or be very nearly empty depending on how long/wet the grass is. The tank looks to hold about about a pint.

[Huge]

Get used to it. The more ethanol they put in it, the worse it is going to get.

[The Natural Philosopher]

No it aint.

I had that when I rebuilt my house. Mower sat there unused for a year and a winter. Wouldn't start or run properly It was about 1/3d full of old petrol. Filled topped up with new and after a few false starts it coughed and spluttered and poured out lots of smoke and then settled down to run as good as ever.

Battery was F88ed and it needed jump starting, but it got there in the end.