This is an all too common with lawn mowers and I will give you some tips on how to resolve this.
A lawn mower that runs and then dies quickly is generally caused by a fuel issue. When a lawn mower first starts, the choke aids the carburetor in getting an extra amount of fuel to get the engine going. While the engine is running in this choke mode, fuel is being routed through the choke circuit primarily.
As the engine warms up or speeds up for engines that have a air vane lever that is moved by the spinning of the flywheel, the carburetor transitions from the choke mode to the main jet.
Here is an easy test. If the mower will run on partial choke and continue to run, then this means your main jet is plugged up and not letting fuel through it and therefore starving the engine.
What needs to be done is the carburetor must be disassembled and cleaned very well. First remove the carburetor bowl and the bottom. It should have a 10mm bolt or a 1/2″. It may also have a fuel shutoff solenoid on it that needs removed and then a 14mm bolt that goes through the bowl.
Then you can remove the float and needle assembly by pulling the small dowel pin that holds it to the carburetor. Set that aside on a clean paper towel. Now depending on what the bowl looks like inside, this will determine if you continue or you buy a new carburetor. If the inside of the bowl has corrosion or fuel that has adhered to the inside of it, this would be one that you want to replace. Also dry and powdery looking residue in the bowl is a sure sign that you should consider replacing the entire carburetor.
If things look fairly good inside, then remove the main jet and emulsion tube that are inside the fuel tower that runs up and down through the carburetor. You will need a standard screwdriver with a thin blade to allow you to get to it.
If you have a brass bolt that held the fuel bowl on, this is your main jet. Now clean these very well with carburetor cleaner or brake fluid and be sure that the tiny ports are cleaned out. You can use a welder’s tip torch cleaner to aid in cleaning them. Just don’t enlarge with the file like end of the tip cleaners. You will also want to clean the needle and seat. A Q-Tip works well to push down to the seat and twist. A soft bristle brush and carb cleaner works fine for the needle.
Spray some carb cleaner into the ports inside the carburetor body and then blow everything out with an air compressor if you have one set to low pressure. Make sure the carburetor bowl has been cleaned out as well and then reassemble everything.
One thing you should do is also take a fuel sample in a clean glass jar to ensure that the fuel is good in your lawn mower and not contaminated with water. It should look crystal clean with a slight yellow appearance and have a strong odor.
After you re-install the carburetor and test run the engine, it should run at a steady rpm with no surging. If it does surge, you can either try to clean it again or opt for replacement. You can also try to snug up the carburetor bolts to make sure it seals well. Be sure to replace the gaskets on both sides of the carburetor if you replace it.
By giving your carburetor a thorough cleaning, it should resolve my mower runs for a while and then dies issue.