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

Dirt in the fuel tank blocking a filter? There's a video on cleaning the carb on your B&S 125cc 450e engine, here

Is this a dry cough?

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?

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

That wouldn't see it do the long grass fine.

There's a video on cleaning

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

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

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."

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.

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

dirt in the low speed jet.

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

Could be muck in the fuel tank blocking the outlet. Or water in the fuel. Or aprblem with the valves (do compression check).

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.

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.

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

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?

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

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

Jeeez - FFS get a decent cordless mower, just a quick blade sharp-up now and again and that's it ....