The motor on my lawn mower starts easily and runs smoothly but after a few seconds it starts to hunt. The plug is sooty indicating a rich mixture but their isn't any mixture adjustment. After cleaning the plug, the air filter and the carburetta it went well for a few minutes but then the problem reappeared.

Hi Alan:
A bit more info would help, like what kind of vehicle, and any symptoms.
Check the inside of the exhaust pipe. sooty is rich, dry is leaner.
Check the spark plugs. sooty is rich, dry is leaner.
Check the mixture. screw in for leaner. If RPM increases, it was rich. If RPM decreases, it is already lean.

You need to reset air mixture jet and also idle speed.. Make sure any dirt particles in carb bowl are removed . Check for answer on U Tube videos,these can be very helpful

Could be fuel or oil fouling. Need to determine which first off. If it is black sooty fouling it is likely too rich also it too rich may be wet.

If oil fouling it is more likely to be much more material fouling and wet sticky material.

A wet, black and shiny deposit on plug base, electrodes and ceramic insulator tip indicates an oil fouled plug. The condition may be caused by one or more of the following: worn pistons, worn piston rings, worn valves, worn valve guides, worn valve seals, a weak battery or a faulty ignition system.

A dry, fluffy or sooty black deposit indicates an air/fuel mixture that is too rich, engine idling for excessive periods of time and/or enrichener usage for excessive periods of time.

A light brown, glassy deposit indicates an overheated plug. This condition may be accompanied by cracks in the insulator or by erosion of the electrodes and is caused by an air-fuel mixture that is too lean, a hot-running engine, valves not seating or improper ignition timing. The glassy deposit on the spark plug is a conductor when hot and may cause high-speed misfiring. A plug with eroded electrodes, heavy deposits or a cracked insulator must be replaced.

A plug with a white, yellow, tan or rusty brown powdery deposit indicates balanced combustion. Clean off spark plug deposits at regular intervals.

check your air filter for dirt or you need to adjust fuel flow at carb lean it out a little

you need to do a cylinder leak down test. That will tell you if it is a valve, which one or if it is rings. Try a compression test too. 5 pulls should be 160 to 190 ish. If all that is ok check the carb to see why it is causing a rich running scenario

If you need further help, I’m available over the phone at https://www.6ya.com/expert/kelly_69a0cf1b13789df1

Black smoke indicates rich mixture. Dirty air filter. choke stuck on. loose main jet in carby. just a few possibilities.

These symptoms indicate a rich mixture. Assuming the carb is the original (and therefore correct) one, and that it has an adjustable main jet, turn in the main jet adjustment screw on the bottom of the float bowl until the engine runs smooth.

If there is no adjustment on the float bowl, the carb will have to be removed and properly cleaned (ultrasonic cleaner recommended).

This is called hunting, your fuel-air ratio is not right. Your carb. jets need cleaned

i advice you to change the spark plugs,this is cause because at cold start the engine needs a rich mixture in order to maintain a smooth idle,before driving the car,when starting from cold ,what you have to do is to accelerate up to 2000 rpm for 3 minutes then you drive it,then it should be ok

Sounds like it's running rich. If the mixture is slightly rich it will start well when cold, but when hot it floods. Check the air cleaner to be sure it's not clogged/blocked. Get a new plug for it and if this happens again just install the new plug - the new plug won't be fouled with fuel. If it started with the new plug then you're on your way to a carb adjustment (that is if the air filter/cleaner is clean)