(This may not be the correct forum, perhaps should post under gardening.)

I have a commercial walk-behind mower with a 12 HP Briggs/Stratton engine. It is 4 years old and began running rough. It acted like it might be caused by a dirty air filter, so I cleaned, then replaced that. I then replaced the fuel filter. I also pulled the spark plug. It was black with carbon, but not thick. I cleaned it and put the old one back in. No luck.

Precisely what it is doing, it starts just fine and runs smoothly. Then when it gets warm, it starts sputtering. This gets worse and worse until it dies. Then it will not re-start.

Please HELP! Any ideas?

Thank you in advance!

sounds like its running out of fuel. need to clean the gell out of the carb and the lines +tank. i dunno what your carb looks like but most of them have a float bowl unless its a different carb. any who you take the float bowl off and clean the small holes with a very small wire . but be carfull as the float needle is there and very small. there should be a small screw and or bolt type on the bottom of the float bowl just remove and take the bowl off in there is the float and needle. look in the carb and you can see the very small hole that feeds fuel to the engine its probaly dirty (got gell )
it may have an adjuster screw on the carb as well sometimes you can open it up and then start the engine and that will clear it . but im sure that you got some pluged holes some where and the only way is to clean them with a very small wire of some sort like a small needle

Agree with Dodge cleaning the carb out is a good idea if its been stood up too long
My method of poking the jets with a wire varies from Dodges way in that I use a full sized wire to poke the large main jet but for the smaller jets with the fine holes I use 1 strand of fine wire (rule is If in doubt if the wire won't go through then use the 1 strand method
They don't take kindly to prolonged poking (very soft metal)
Give the float chamber a complete clean out Has to be spotless remove the float and wipe the float valve seat. Reassemble the Carb and prime it by turning the engine over without the spark plug in may help?(ignition off!)
Now connect everything up and you should get a first time start
Still sounding sick? Then you'll need the engine HandBook with a coked spark plug bad starting the answer lies in how the pilot air screw is set on the carb Hope its not sealed (as in road vehicles)
In the hand book it should say how many turns this screw set at and its very fine There are 2 screws 1 for the idle and the other in the carb body(the air or pilot screw)
Vibration can loosen this though on some setups there is a spring to stop this happening only very small so don't undo it
Instead with a fine screwdriver screw it in until it stops then unscrew it exactly to what it say in the book Might be 1 turn or 1¼ turns out In my experience 1½ turns is about the maximum never more than 2 turns!
If you haven't got a handbook and the engine is starting & ticking over get your (fine)screwdriver and slowly turn the screw out and if the idle picks up that is good . If it fades then wind the screw in or increase the tickover on the other screw...The secret is to balance these 2 adjustments only finely too much The engine will race or it stalls! ( I'd rather have it do the second thing!)
Set it @ 1½ turns out to begin with and the best setting will be a ½ turn max either way
Check the spark plug gap too and if the electrode looks flakey buy a new plug
4 years service on 1 plug? That's maybe contributing to this issue?

quote:

4 years service on 1 plug? That's maybe contributing to this issue?

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Plugs can look good and run good at first, but when heated up they can short out...Spend a buck two ninty five and put a new one in.....
Also, run some carb cleaner thru.....

Carb cleaner?
Redex will also clear the soot out of the engine
Only need a capful in the tank otherwise the engine will smoke like crazy add every few months
I think its sold as a different brand name in North America But I find its the best value (worth looking out for )
Here are some others anything look familiar?

agree change the plug every year and when you park it put fresh fuel then after three weeks dump that fuel fire it up and let it run out of fuel then fill it with fresh fuel. if you do that you will never have a problem thats fuel related (except the ajusting part )

You Did Not replace the spark plug? If not replace that first. The older they get the more prone they are to be affected by heat.

If that fails to work, then try the carb cleaner a cap in the gas tank as stated by Bedstor. However since you are going to have a problem keeping it running I would strongly suggest you drain the gas tank first, then get fresh gasoline (Gas pumped from the station that very day) and try that with the carb cleaner. Old gas is degraded or can have water in it could be part of the issue.

I have another chore for you while you are draining fluids, drain the oil and replace – I didn’t see any mention of that. Sluggy (dirty, thick) oil can cause the engine to do strange things on smaller engines like lawn mower engines when they get hot. Thicker, or dirty oil can also cause undue wear and tear too.

You didn’t say if she is blasting out any smoke when starting (other than the usual first belch), nothing about strange smells. Nothing about smoke when she starts sputtering. Right? So we can assume that there is not too much gasoline or oil getting into the wrong places.

If that doesn’t work – I would suggest taking it in for servicing.

Unless you are good with carburetors and are comfortable with poking and prodding with one.

The carburetor may need just a few minor adjustments, or it may need a deep cleaning, or it may need a tiny little spring replaced, or it might need…. I think you get the picture.