lawnmower (petrol) engine – fix it or buy new?

got a sovereign NG460 if that means anything to anyone, bought it second hand from a mate for a fiver last year, so no complaints about that! its got a briggs and stratton ‘reliable starting’ 35 classic engine on it.

however, it doesnt run sweet. if you leave it ticking over its fine, but when you mow the grass, every 30 seconds or so it dips in power almost to the point of cutting out, so we have to stop, wait for the revs to catch up again and go again.

mrs exp is already browsing the internet, looking for a new one, but id like to explore the option of making this one run better. especially as i hear B&S engines are good quality, and what we’d get from a shop for just over a ton maybe inferior.

i know nowt about engines, but if you chaps tell me it needs a new spark plug, filter or something cleaning out, then thats worth a punt to me. anything more complicated and i may struggle 🙂

thanks a lot

Service what you’ve got, unless you like contributing to landfill ?

A new filter will always be a good idea, then take out the sparkplug and check its condition – that can tell you a fair amount about how it is running.

Changing the plug and overhauling the carb fixes most problems with a brigs motor. Parts cost about a tenner

Service what you’ve got, unless you like contributing to landfill ?

Tricky one this. Normally I’d be all for repair as the environmental option but petrol mowers are terrible in general for the environment and a poorly running old one especially so.

Going back to my karting days here but I’d suggest a new plug and a carb rebuild. Should sort out most problems. For something that popular there should be loads of videos on Youtube on how to do the carb.

What passes for a carburetor on a Lawn Mower is a pretty simple device so I would also suggest taking it off/apart and cleaning and checking over the carb’

Check that the throttle plate moves through it’s entire range and also check that the ‘throttle’ cable moves freely and has the intended full range of movement.

Replacing the spark plug can only help – so I’d do that as well.

In most areas there will be someone who buys up non-runners from boot sales and the like, refurbs them and flogs them on Gumtree. Have a look on there for someone selling more than one, or with a history of repeat sales. They will be happy to service it for a lot less than someone who does it as a full time job. If you were in South Yorks I could give you numbers of a couple of guys.

thanks chaps, just been looking at a few carburettor vids on youtube. all they seem to do is change the diaphragm and gasket, nothing else to it?

ill probs have a look at mine, see if i can start stripping it and what the air filter and plug look like too.

ebay the best place for new plug/filter/carb parts? and what sort of oils in there if i change that? oil filter too? not sure how much to do to it for a service……

thanks a lot

Agree with above. Get the carb off and take it into the kitchen, some isoproyl alcohol and clean it right up. Check the gaskets are ok. Clean the fuel/replace the fuel lines, tip out the fuel tank and clean it, fuel filters (mesh cap on the end of the fuel line inside the tank), pop a new plug on, replace the engine oil.

If it still acts up after this, just get it into a local machine shop and ask them to specifically sort that issue out. It’ll be worth it I’m sure.

Normally I’d be all for repair as the environmental option but petrol mowers are terrible in general for the environment and a poorly running old one especially so.

i agree in principal but given that the mower will see what 30minutes use once a week/twice a month…. how long till it balances out with the materials/energy needed to make a new one and ship it all the way across the world…..

also my parents had this on a briggs and stratton powered mower.

was simply a choked up air filter.

REmove filter and run it (but dont go cutting grass or it will suck the grass in…) and see how it is …. theirs ran fine so we rplaced the foam filter and its still running today. 10 years sterling service – far better than the
plastic fantastic electric mower that didnt last a season.

Given most mowers dont get serviced – they are pretty robust.

mine was doning this for a bit last summer. Cleaned carbs and plug etc etc, eventually Got some EGR cleaner from Halfords and gave it a blast, lots of black smoke and engine running like new now. (coked cylinder).

REmove filter and run it (but dont go cutting grass or it will suck the grass in…) and see how it is ….

problem with this is, its only when cutting grass that it dips the power, so running it on the drive wouldnt show whether thats the problem really.

ill start taking it to bits, whats the worst that can happen 😉

Where are you?

If you’re in Nottingham, I’ll happily whack the carb in my ultrasonic cleaner for you.

I love tinkering with and rebuilding small Honda generators – the usual problem is fuel being left in them over time. This leaves a hard layer in the carb jets. Ultrasonic cleaning is usually the only way to clean them properly.
Other than a carb rebuild it’s usually air filter, plug and oil change.
Empty the fuel tank when not in use though even if only for a few weeks.

try this

A couple of years ago I bought a John Deere mower that wouldn’t start for £30. No compression and pulled over too easily. I took the head off and found that the exhaust valve seat had come away from the head, so I put it back in place and peened around the edge (as per internet advice) to hold it in.
It’s still working great and is worth 5x what I paid for it!

People are far too quick to throw stuff like this away – the engines are simple and quite fun to play with. As you said punky – what have you got to lose?

I bet you fix it.

good link sharky, thanks. ill try and check as many of those as possible with my limited knowledge/skillzz.

Where are you?

If you’re in Nottingham, I’ll happily whack the carb in my ultrasonic cleaner for you.
im in lincoln mate, so not toooo far. very kind offer, if i cant fix this i may give it to one of my workmates who lives in nottingham (cotgrave) to pass on. thanks.

started with the stripdown. took the air filter off, didnt look tooo shabbly…

until i turned it over …….

so that could well be the culprit, altho now im on a roll i want to do what i can.

taking the petrol tank/carb off, i realised i hadnt noticed where one of the springs went. can you enlighten me? its the real skinny spring at the back. doesnt matter too much at the mo but i need to know for when i put it all back. also the ‘moving part’ fixed to that bigger springs pretty stiff, im sure that spring wouldnt move it on its own if thats whats sposed to happen. dont think its related to my problem but is it all sposed to move really freely?

carb off and i looked at the diaphragm and thought hmmm that looks ‘blown’. but then when i took it apart i realised the diaphragms really thin and theres a spring pushing onto it, so thats normal yep?

ill certainly get a new kit for it anyways, just not sure wheres best. ebay?

and is it ok to give the carb a good clean in soapy water? i know sobrietys ultrasonic cleaner would be better, but is water a half decent substitute or does it want to be soaked in petrol or something?

thanks

Assuming that the engine isn’t overheating and seizing (it has got oil in, hasn’t it??) and that the speed control vane that works the throttle isn’t jammed, it’s likely muck in the carb, or a failed diaphragm/gasket.

(BandS carbs are “odd” – the diaphragm also acts as a fuel pump. May not run at all without the air filter on.)

FWIW, I’d replace the diaphragm (£5 ish)and clean out the carb while I was at it.

[EDIT] Just seen your post above – don’t clean the carb with water. Use diesel / meths / white spirit / proprietary cleaner or petrol (carefully).

Replace the diaphragm – our local mower place keeps them in stock (no good to you, though!)

this is the little spring that i dont know where it went

just taken the spark plug out, looks black and charcoaly/oily. havent got a set of feelers for gap, but can get access to some easy enough i should think.

From your OP I thought “it’s running too rich.”

Looking at the plug I thought “it’s running too rich.”

Looking at the air filter I thought “and that’s why.”

(I am not a mechanic and know nothing about lawn mowers.)

That looks very, very rich. I’d say cleaning the carb and replacing that horrible air filter will sort it out.

That or the piston rings are gone and it’s burning oil, which is the other reason it’d be struggling. But Carb/air filter is more likely to be honest.

As to the spring, no idea. Did you take pictures of it in situ before you started pulling it to bits?

Edit: What he said. Also, if it were burning oil you’d have blue smoke coming out of the exhaust and an oily-burning smell to go with it.

T’interweb usually has a pdf manual with exploded diagrams in if you google the model number of the mower/carb…

As to the spring, no idea. Did you take pictures of it in situ before you started pulling it to bits?

i went out with the best intentions of doing so, but er….. 😳

i went out with the best intentions of doing so, but er…..

We all do it, some of us more than once…. 😳

New plug time!

Remember to soak the new air filter in oil then squeeze out the excess- foam alone doesn’t filter

New plug time!

Remember to soak the new air filter in oil then squeeze out the excess- foam alone doesn’t filter

ok, ill get a new plug. i soak the filter in oil? an air filter?? id never have guessed that, sounds to me like itd clog the ickle holes up.

this little ‘well’ in the casing….. had some shite in it. scraped a lot out with screwdriver but still a bit dirty. thought it was a filter at first but its solid metal i think. whats that for, just a ‘holding area’ for fuel? and does it need to be squeaky clean? the pic below is after id been foraging, but obviously cant get it spotless with a screwdriver.

thanks a lot

Squeaky clean, all 3 of those holes/bowls/basins.

so ill have to bin the petrol and soak the whole casing in something? soapy water? petrol? WD40?

thanks

The petrol is probably ‘off’ modern fuel doesn’t like being stood, as the ethanold content absorbs water and rusts things.

You might be lucky, but for the couple of quid a tank of fresh stuff costs, it’s worth it.

To clean the carb I’d use carb cleaner, brake cleaner or wd-40 in descending order of ‘goodness’

All good advice so far.

That small spring looks like it is part of the throttle arrangement. It will probably clip onto a plate that is moved by the throttle cable and will cause the throttle to return to a lower operating speed when the governor (within the pull cord assembly) says the crank is turning fast enough.

Plug gap looks massive. Also with no lead in petrol these days, you can’t rely so much on plug colour….but that to me screams running rich and I’m sure a clean up, new air filter, new or cleaned plug and fresh petrol will have it running as new. Change the oil whilst you are at it.

Soak the filter in clean engine oil and squeeze the excess oil out. You can get specific foam filter oil but engine oil is better than nowt.
The oil traps the light dusty stuff that would pass through the pores of foam.

Are you wearing a suitable shirt while working on the mower? 🙂

This might help you with the small spring

Briggs by mtbjeff[/url], on Flickr

So you set the throttle, and then the pneumatic governor works based on wind pressure off the fly wheel. Big load = no wind = more gas

FWIW, my mower came off freecycle – carb needed a good clean and a new plug and has been fine for 2 years now.

tbh While in the past I’ve rebuilt 2-stroke engines I CBA with our lawnmowers 4-stroke so just take it every other year to a local horticultural engineers and they service it plus stick a new blade in etc.

Normally I’d be all for repair as the environmental option but petrol mowers are terrible in general for the environment and a poorly running old one especially so.

What should I use instead, and electric one – got one with a 100m cable?

This might help you with the small spring

lovely, thanks.

Are you wearing a suitable shirt while working on the mower?

i do not have such attire. shame 🙂

this the service kit id need?

with this diaphragm/gasket set?

thanks

When the diapragm leaks, the engine runs **very** rich under load (enough to stall it eventually).

Air filter won’t help, either!

If you soak the foam in oil, you need to squeeze it out after. FWIW, I would just put a teaspoon full of oil to the cleaned filter and massage it to spread the oil around.

Insides need to be squeaky clean. Don’t use water, or it’ll be a bugger to get it out of the small drillings and jets in the carb. That recess is part of the fuel pump.

Not quite up to Mowgli standards but you’ll still be well chuffed when you mange to repair it yourself. 😀

PS – that diaphragm is goosed, under load you are using more fuel yet with that diaphragm the carburettor cannot lift enough fuel to supply the demand, and that filter will be choking the engine thus making it lift even more fuel and spitting it onto a huge plug gap that eventually oils up the cylinder.

Looks like the right kit you’ve listed.

That engine looks the same as the one in my mower. The diaphragm kit you linked to (795083) is the same one I have purchased. If find it fails every second year and gives a steady rise and fall in revs called “hunting” when its perforated.

I wouldn’t bother with the new sponge just wash in soapy water and re-oil. I just use whatever I have typically 5W30. Do this every year. Fresh fuel also helps especially if its been stood all winter. A replacement spark plug would be code RJ19LM.

the filter comes with a kit thats cheap enough anyway mate, so i should be able to get everything i need for under £20.

just rang a local lawnmower shop who says he should be able to supply the lot for around £11 so ill go see him tomorrow.

thanks a lot

Next step… this[/url]

Did you bloody well have to? I’ve just had to go and cut the grass in the spring sunshine after reading this… 🙂