Replaced carburetor but lawnmower still not starting

Try running with fuel cap off. Might just be bunged up

Prime it with the fuel filler cap off, I've found that helps when it hasn't been used for a while, stops any vacuum building up in the line.

A new spark plug and fresh fuel has yet to fail for me.

Stupid E10 maybe? I used this to my last can of fuel last year. Was running shite all year on old petrol. Hopefully will start in a week or two.

https://www.screwfix.ie/p/mountfield-ms1211-universal-fuel-stabiliser-100ml/6744J?tc=BI5&gad_source=1&gclid=EAIaIQobChMI4PHtuvaHjAMVA5RQBh3PdSYVEAQYAiABEgJ0z_D_BwE&gclsrc=aw.ds

My mower last year was getting petrol but the fuel line was bunged a bit with the newer petrol. Was actually perished inside.

A new line sorted it

OP, remove the bowl (one 10mm nut) then remove the float - push the hinge pin out. Then clean the end of the float valve and if you can clean the float valve seat (ignore if you don't have anything suitable I use a interdental tooth brush). Reassemble but do not overtighten the bowl nut as doing so can deform the bowl making it leak at the top seal.

A sticking float valve is typical of the symptoms you describe and very common after storage. On a new cab its probably just poor tolerances (cheap chinese).

I get you, but it seems mad to me.

Better practice would be to run the fuel out at the last cut of the season.

Well OP we all want to know what if any fix worked ;-)

The float can still be moving freely and the needle is getting stuck.

Try removing the needle and cleaning it off and the channel it runs in. Try not to damage the seat.

/goes back out to cut some more grass ;-)

For me if fuel is OK then I'd replace the spark plug. I have had plugs that will start an engine OK but then not run it once under pressure and warmed up.

If you want to double check on the fuel then get a liter of Aspen fuel bigger mower shops and even some builders merchants sell it. It doesn't go off and has a cleaning affect. Obviously get rid of any other fuel in the tank first.

Ah… Just thought, not had this issue in years but when I had a gang of lads working for me broken flywheel pins were a regular occurrence. It doesn't give the symptoms the OP reports but its a fix that is often missed.

Edit> Just thought I'd add my experience is that even the slightest damage to the flywheel key will massively screw up the engine running. If on removing the key is dented even the smallest amount thats the problem. Sometimes you need to remove the flywheel to see the damage on the key. On Briggs and Stratton engines the key is made of aluminium.