Briggs and Stratton lawn mower engine started coughing, spluttering and backfiring, and stalling on the slightest load

I have a Spear and Jackson lawnmower with a 125 cc Briggs and Stratton engine (this one https://www.argos.co.uk/product/7597004 ). After a year's perfect service, the engine has suddenly started coughing, spluttering and
backfiring, and sometimes repeatedly stalling as soon as it tries to cut a trivial amount of grass (paradoxically, it cuts longer grass without batting an eyelid).
At first I thought it was running low on fuel, because the uneven running sounds identical to that which leads up to running out of fuel. But there was plenty of fuel. And the fuel is about 1 month old - it's not been standing around for ages for its most volatile fractions to evaporate.
I found several things that *could* have been the cause - but weren't. There was a build-up of grass mowings stuck to the underside of the mower, which could have been making the blade harder to turn - fixed that but the problem persisted. The air filter could have been blocked - the sponge filter looked clean but was dripping in oil, so I cleaned it in hot soapy water and gave it a *light* coating of oil as the manual says, but to no avail. The sparking plug was coated in a thin layer of black carbon, but the gap looked OK and there was no coking-up of the contacts - I cleaned the carbon off the underside of the tag and the end of the rod (ie the surfaces between which the spark jumps) but that didn't help either.
I've got it to a state where the mower will usually run for tens of minutes, with irregular misfiring and occasional backfires, but will suddenly stall without warning, usually when cutting short sparse grass, rather than when it's working harder to cut denser or longer grass. And once it starts stalling, it does it repeatedly. My impression is that it seems to be using a bit more fuel and that the smell of the exhaust is stronger, so could the mixture be richer? The regulation of speed (ie the governor) seems to be as good as ever: when the mower encounters tougher grass, the engine note gets louder but not slower. I've tried with the drive to the wheels disengaged (ie I have to push the mower) in case that was a factor - but it made no different.
Any ideas? Is it time to "get it looked at"?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Dirt in the fuel tank blocking a filter? There's a video on cleaning the carb on your B&S 125cc 450e engine, here
https://www.youtube.com/watch?v=v_4MurQyBgU

--

Chris

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
<snip> >I've got it to a state where the mower will usually run for tens of minutes,

Valve gaps? As the engine heats up and stuff expands the valve gaps get smaller, the engine heats up on the longer bits and so appears to misbehave when on the shorter stuff? <shrug>
Take the rocker cover off (often easy on such things) when it's hot and check you still have play on both valves (you make have to turn the engine over a bit to check both valves (make sure it's off, pull the plug cap off, keep hands away from everything etc)).
You often need around 10 thou gap (they can be different). Easy to check / adjust yourself but you will need a feeler gauge set (cheap) and a spanner / screwdriver.
Cheers, T i m
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

This gives you and idea of the sort of thing you might need to do (and he explains the background a bit as well).

https://www.youtube.com/watch?v=U6NdqHHldUM

Yours could be completely different in the details (self locking nuts, different gaps etc) but the principals / processes will be very similar.
Even if the gaps turn out to be right (typically set cold), at least you can them rule them out. ;-)
Cheers, T i m
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 23:49:20, T i m wrote:

In which case the engine is more likely to stall under heavy load; where the valve stems will expand more with temperature.
The OP said "will suddenly stall without warning, usually when cutting short sparse grass, rather than when it's working harder to cut denser or longer grass."
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

How fast do you think metals expand under such circumstances?

I know and had already accounted for that (see above).
In any case, it might not be a bad idea to check the valve gaps eh?
Cheers, T i m
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
wrote:

That wouldn't see it do the long grass fine.
There's a video on cleaning

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 22:16, Chris Hogg wrote:

I would clean the carburetor, particularly the jets. New ones are quite cheap on ebay.
--
Michael Chare

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 20:53, NY wrote:

Is this a dry cough?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 20:53:08, NY wrote:

Sooted spark plug does suggest carburation issues.

Which suggests mixture is perhaps closer to stoichiometric with higher loads.
Does it tick over / idle without misfiring?
I would be tempted to follow the Youtube video for servicing the carburettor that has been posted on this thread.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The engine misfires under all loads, from idling (blade turning, but clear of grass) up to heavy load. It's the intermittent stalling that seems to happen a lot more frequently under light load (but not idling).
The suggestions about the carburettor seem very plausible - particularly to the mixture being too rich, given the increased fuel usage and the stronger smell of the exhaust (incomplete combustion?).
Would over-rich mixture (without an accompanying problem with the governor at light loads) explain the symptoms?
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

I would have thought that most carb problems would lead to under-fuelling rather than over-fuelling. I think it would be worthwhile checking the flywheel alignment on the crankshaft. I’m not giving up on my mistiming theory. ;-)
Tim
--
Please don't feed the trolls

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Odd one. Could you have hit a large rock back a the start of all this? I’m wondering if the ignition timing is now out due to the flywheel having slipped on the crankshaft. It’s not unknown for the woodruff key to shear.
Tim
--
Please don't feed the trolls

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 20:53, NY wrote:

dirt in the low speed jet.
--
Karl Marx said religion is the opium of the people.
But Marxism is the crack cocaine.
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
The Natural Philosopher presented the following explanation :

Back in my carburettor days of driving cars, a trick of mine to easily clear jets, was....
To remove the air filter, get the engine revving hard- then slap a hand over and blocking the air intake. It puts maximum intake suction on the jets, to suck any dirt through. It didn't always work, usually it would, but it's a quick fix.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Saturday, 11 July 2020 21:02:36 UTC+1, NY wrote:

Could be muck in the fuel tank blocking the outlet. Or water in the fuel. Or aprblem with the valves (do compression check).
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Sun, 12 Jul 2020 18:59:25 +1000, "Rod Speed"

Except, things like that can start to be a problem *once* the engine has fully warmed up.
Further, our car would start (/restart) when hot or cold, not when warm, but would run perfectly well at all temperatures, including warm.
I did sort a motorcycle for someone that gained an airleak on the induction side on one carb once warm, only after the local bike shop had charged him to strip, clean and re-jet the carbs ...
A batch of 4/ strimmers were losing power when run hard for a long time but not if only run hard between periods of ticking over. That was down to the valve gaps closing up ...
Cheers, T i m
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 20:53, NY wrote:

Sounds like over fuelling. Too much fuel at low loads.
This will be one of two things either the gasket for the primer has failed or the float valve needle has stuck open.
M<y money is on float valve.
Mike
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On 11/07/2020 20:53, NY wrote:

Jeeez - FFS get a decent cordless mower, just a quick blade sharp-up now and again and that's it ....
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.